In addition to the answers already given, something to keep in mind is you are not the only one with dailies to spend. Thus, the question is not so much when should you expend resources in a fight, but rather when someone should spend them. For example, the party I play with leans heavily towards single or double target Strikers as primary or secondary roles. They use their dailies when something needs killing now (usually when it is obviously the baddest thing in the room), but those dailies can largely only deal with single targets or small groups. As the sole Controller, I use my dailies when we are up against a large group, because it allows me to break it up into manageable chunks for everyone else to deal with. Thus, I hang on to my dailies until either the party has agreed to rest after the encounter, or we are up against too many foes to take one-on-one.
That said, I also favour the panic-button response; start with the smaller-stuff (Encounters, At-Wills), but if things get ugly pull out the big guns. There is a lot to be said for carefully husbanding resources, but taking out a threat before it can cause much damage to the party also saves resources, specifically healing surges (or resurrection costs).
EDIT: I would also add, if you have something that lays down a large-area sustainable or permanent zone, use it on the first turn if you are going to use it at all. That way you can benefit from it the whole encounter, and if it is a damaging zone it'll pop the more unlucky minions before they get a chance to actually do anything nasty. Giving half the critters on the battlefield a good once-over in this way lets you sort out which things are tough enough to need focusing on for round two, and can very often end encounters before they get going. If the area is unfriendly, it also lets your allies work around the spot where it is going to be, rather than already being in the thick of it when you drop it on their heads.