Can you have a 1 round cantrip always active (prior to initiative)?

Question

I'm planning on doing some scouting and wanted to know if I can tell my DM that I always have BladeWard up until something happens (either I'm surprised or see something - basically when initiative needs to be rolled).

BladeWard
Abjuration cantrip
Casting Time: 1 action
Range: Self
Components: V, S
Duration: 1 round
You extend your hand and trace a sigil of warding in the air. Until the end of your next turn, you have resistance against bludgeoning, piercing, and slashing damage dealt by weapon attacks.

Will this impact my movement? I do plan on doing this from the air (aerial recon). I'm a level 3 rogue so I have a bonus move action if that makes a difference.

I suppose if I take an action to cast the cantrip I may not have an action to make an active perception check, but I should still have a passive check for hidden things.

Any support RAW for or against this would be great.

Answer 1

Yes.

On your turn, you can use your action to cast a cantrip that you know. There's no limitation on how many times or how often you can use your cantrips. Your movement will be impacted only in that you will not be able to use your action to dash (but, as you identify, you can still dash with your bonus action).

That covers what you asked about in the question, but here are some other concerns that might arise as a result of casting a cantrip every round:

1. You can't use your action for anything else. You may want to use your action to make a perception check, to dash, disengage or hide on top of what Cunning Action allows, or to interact with more than one object in a round. You won't be able to do any of these things in a round where you cast Blade ward.
2. You need a free hand for the somatic component. Unless you have the War Caster feat, you need one hand free to perform the somatic components of the spell. So you could not hold two weapons, or a weapon and shield, or a weapon and a wand of fly or whatever while casting this spell. A two-handed weapon is okay though- you can hold it in one hand while casting.
3. You need to speak the verbal component. You may alert enemies to your presence when you speak the verbal component of the spell. There is basically no guidance in the game about how loudly you need to pronounce the spell, so talk to your GM if you are concerned about this.
4. You might scare people. Casting Blade Ward every six seconds suggests that you expect that you might be attacked in the that period of time. This is probably fine while you are doing aerial recon, but potentially scary and off-putting if you are doing it while shopping in town. Also note that many NPCs may not be able to identify what spell you are casting. From their point of view, it could just as easily be Dominate Person or Fireball.
5. You may annoy your fellow party members. I'm just imagining spending all day next you a guy going "blade ward... blade ward... blade ward..." ALL DAY LONG. It sounds obnoxious, but this is a roleplaying opportunity more than a reason not to do it. It definitely says something about your character if you are the type of person that would want to do this.
6. You might be distracted. I don't think that there is any RAW support for this idea, but anecdotally some GMs might decide that you are focusing too much of your attention on casting the spell, and give you a penalty to perception or social skills as a result. Again, talk to your GM.

Answer 2

Yes you could but...

It would affect your movement but only to the point where you would be moving at the same rate as everyone else since you would not be able to Dash twice that round as a Rogue (Cunning Action and Normal Action both using Dash). However, it is worth noting that this spell does have a verbal component and those have to be spoken with some force therefore this could affect your stealth roll.

The words themselves aren’t the source of the spell’s power; rather, the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion. PHB 203

Your DM may differ with this but the italicized verbiage as well as the metamagic from Sorcerer seems to imply you can't "whisper" verbal components.

Essentially, every 6 seconds you are announcing your presence to potential ambushers.