Your GM needs to ask you some questions.
Here's somewhere to start from: there's usually no drawback to applying touch poison to a weapon at the moment you need it, except in circumstances of extreme danger. If you want to hedge against that danger, you'll risk wasting the poison or accidentally applying it to something else, unless you find somewhere to get custom gear made to work around the downsides and pay extra for it.
How did I get there? It's a mix between conversations I've had with various players about their concerns over touch poisons and a little application of the game's principles and moves. Questions asked and answered, which is an important part of being a Dungeon World GM.
But if you're a fairly new player to the system, with a fairly new GM, they might not know what they should be asking and why. So here's what I'd ask and why, and the answers that built up that "somewhere to start from".
First: "What are you worried about?"
This is kind of an address the characters, not the players moment? It's absolutely important to have a conversation about the choices you make during character creation, but Dungeon World isn't a game about independent rules elements coming together, such that you can establish an absolute meaning of any one of them and move on. If you want to know how something's going to play out in a scenario of concern you'll be best-served by expressing it as a concern Shanksworth the Thief's would also have about using poison. In my experience, most character-facing concerns come down to questions of time and safety.
Time: "I'll see an opportunity, but it'll take too long to put the poison on." This might be inspired by the example of play, where Omar gets one narration pass just to soak part of his cloak in applied poison to feed to a crocodile? Well, the thing about both of the touch poisons is they're not applied and they're also not slow, and why that matters is a question of-
Well, it was better stated in some later games, that there's this concept of the spotlight - the GM isn't having a simultaneous back-and-forth with everybody at the table at once, but going around in turn to give each character a separate chance to act. While they're acting, they have the spotlight, and they hold onto it for as long as it makes sense -- generally this will be until they do something interesting or time-consuming. Slow needs time and applied needs care, both of which imply something time-consuming, but just getting poison ready to use isn't, of itself, very interesting.
This is a be a fan of the players' characters concern, specifically "give them what they've earned". Picking a touch over an applied poison to start with includes this question of speed and focus - if the GM is just going to make every touch poison work like an applied poison in practice, what's the point of touch?
"I won't take the spotlight off you just for using a touch poison" is a reasonable position to take.
Safety: "If I'm in a situation where it's risky or dangerous to move enough to apply poison, I'd like to have a poisoned blade ready to go instead." A lot of these concerns are kind of parallel to time concerns - most of the things that would expose you to danger are things where time or care are a concern, and time and care aren't really a concern for touch poisons.
There's a broad category of situations where you're in constant danger because you're being immediately menaced by a monster, like if an ogre grabs you up. But in those situations, remember that touch means it just has to make contact with the skin, not that there has to be a wound. You're in kind of a broad-side-of-the-barn situation there, at least as far as applying touch poison goes.
That said? I'm not gonna lie, if you're in a situation where you have to defy danger and it's interesting enough to be the whole of what you do, I'm gonna be real real tempted to give you a choice/bargain about either applying your poison or dealing your damage.
So that brings up the question of:
Second: "What are you planning?"
This one's really a combo slam of ask questions and use the answers and tell them the requirements or consequences and then ask, maybe mixing in a little show a downside to their [...] equipment for spice. How are you planning to keep potent poison on a blade for a week? Will the blade be covered - sheathed, wrapped, etc? How do you keep the poison from rubbing off on the cover? Will the blade be exposed? Well, the poison's not dangerous for you to handle, but the same can't be said about anyone or anything else it accidentally comes into contact with.
Those are potential consequences, not inevitable ones. They're the downside of trying to handle this problem with your current equipment. They might never come up, at least not if you invite Hearts Boxcars over to be a ringer for your dice-rolling. (The thing about Hearts Boxcars is he always rolls boxcars.) They're not inevitabilities, things you the GM are making up out of spite to stop the player from doing what they want. They're like the wraith in the catacombs guarding the arm of St. Barsabius -- something players will have to work around to get what they want, or tough through and maybe get blocked.
One way to work around the limitations of basic gear, especially in cases where there isn't some tagged item that'll serve as a ready answer, is the "custom item" listed as a service in the equipment section. It's explicitly called "a custom item from a blacksmith, base price +50 coin" but I see no reason why it can't apply to other sorts of crafter as well. In addition to the extra cost in coin, an item like that will probably need a Supply roll to track down, unless it's been established that you're in like a thieves' guild town where all the merchants sell implements of trickeration.
You want a knife with a modified blood groove that'll hold onto a dose of poison until a stab connects? A custom-fitted sheath that maybe needs a little bit of wax on the knife handle to make it into a poison vial with the knife as the stopper? They'll keep a dose of poison on a knife as long as you want it there, but at a cost in coin and opportunity over your standard gear.