The logic works like this: You have four cards, but not all of them are usable. You can't use the Daily, because you might need that later. You shouldn't use the At-Will powers, because they always give you the least bang for your action-buck. The only card remaining is your Encounter power. Thus, you should open up every fight with your Encounter power, and then go back to your At-Will powers for the rest of the fight. When you gain your second Encounter power, you then have a choice between which order to use them both in, before going back to your At-Wills (and continuing to ignore your Daily powers).
If you care about the action economy, then it's logical to maximize the effect of each action. It's possible that the best course of action would be an At-Will instead of an Encounter power, but At-Will powers are at an inherent disadvantage in that comparison since they have a smaller design budget.
Thankfully I've never encountered this problem in my 4e gaming, because my players make their assessments of rational actions based on the actual situation their PCs are in.
For instance, at 1st level the fighter's encounter power was Passing Attack, which allows an attack, a move, and another attack against a different target. There is no point using this power unless (i) there are two targets to attack, or (ii) you really need the free movement. A further consideration is that every attack the fighter makes lets him mark a target, so that a factor in relation to (i) is whether or not
now is the time to mark two targets.
At the same level the wizard's encounter power was Icy Terrain, which is an AoE that knocks targets prone and creates difficult terrain. This power is best used when (i) the enemy are grouped together, and (ii) the PCs will benefit from controlling the enemy's movement (standing from prone costs an action, and difficult terrain reduces movement speed).
Sometimes it is best to open with these powers, but not always. And the number of combats in which it is best for both to be used in the opening round is even fewer - for instance, if Icy Terrain has been used to hold off a chunk of enemies, the fighter might be better engaging the NPC in the van with at-wills while the ranged strikers shoot or blast the others slip-sliding around on the ice.
If in every case players take encounter powers that are nothing but damage-buffs (at 1st level, the closest to this for a fighter would be Steel Serpent Strike: 2W, slowed and cannot shift; for a wizard, it would be Chill Strike: 2d8 and dazed) then perhaps it's always best to lead with them, but frankly if you build a boring PC, complaining that it's boring in play seems like reaping what you've sown!
The PCs in my game are 28th level, about to go to 29th. I've attached a 27th level power sheet to this post, for the invoker/wizard. This character uses At-Will powers in combat (most often Hand of Radiance for multi-target, but sometimes Mantle of the Infidel for range). Working out when to use which power, and co-ordinating that decision-making with the other players, is an important element of playing the game.
Obviously play was simpler at 1st level (many fewer choices) but I don't ever remember it being simplistic.
For improvised actions, you have to factor in some penalty to their effective efficacy based on uncertainty about how it will resolve. If I'm remembering page 42 correctly, though, most improvised actions should be balanced with Encounter powers. If that's the case, and if you're fairly confident that the DM will use that guideline, then it might be worth switching to improvised actions as soon as you run out of Encounter powers.
Well, this takes us back to the issue of illusionism, doesn't it!
Why would a 4e GM not be using p 42? - those are the rules of the game! In my personal experience, especially once the suite of powers open up, players use p 42 to enhance their powers rather than substitute for the - eg they try and push people into damaging zones, or impale or trap them, etc. (Even the example in the DMG is like this - the player uses a Bull Rush - which is an at-will power that all PCs have access to - to push an enemy into a fire.)
In my game, when we resume our next session it will be Pazuzu's turn and - being dominated by the invoker/wizard - he will fly into an Abyssal rift to the Far Realm. Time to apply p 42!