From a personal perspective, I don't see this being unrealistic. Back when I studied martial arts, I could size up an opponent pretty quickly, by the time the first few punches were thrown if not before. You can see it in their eyes, their stance, the way they attack. It's a necessary skill if you intend to do well when sparring (assuming one hasn't been burdened with an overabundence of natural talent in which case it's simply helpful).

And I'm no adventurer.
That 5 dagger tactic seems to me like it would be sub-par. In return for a full round of the PCs attacks they dropped 2 minions and did minimal damage to a non-minion? If I'm running the combat the remaining 8 will take up tactical flanking positions on the PCs (preferrably the less armored ones) and proceed to beat them senseless. It's almost as good as giving the enemy a surprise round.
Most characters should be using their at-will abilities on the first round, inflicting a reasonable amount of damage/ debuffs on them while feeling the enemy out. The defenders should be getting in the grill of the biggest, meanest looking enemy they can see and marking them. It's the wizard who should be trying to play "guess the minions" so they can burst their bubbles. I admit that I haven't seen it attempted, but I can't see the 5 dagger opener being especially effective.
Of course the characters won't use it against the group of 4. One of the assumptions of D&D (even moreso than in the past, it seems, in 4e) is that parties fight monsters that aren't too dangerous or too easy. If anything, I'd say the metagame assumption is that when the party comes across 20 bandits or a BFD (big frikkin dragon) they initially assume they can win the fight, assuming the DM isn't dropping hints to make them think otherwise. It's almost silly that, after stopping the BBEG's plans, he doesn't send some uber-monster to destroy the PCs (something 10 levels above them). But, since it's a game and that wouldn't be fun, we assume that the BBEG is busy with other matters, or mortally incompetent/ arrogant, or what have you.
It's your game and I'm not trying to tell you how to play it. I'm just somewhat doubtful that your "fix" will remedy the "problem".