Well, no need to pile on the "wolves use smart tactics" bandwagon. So...
I assume that a monster has whatever instincts are required to use its abilities in a generally sound fashion. If something has a breath weapon, for example, it will try to blast a group of opponents in preference to a single target; even with animal intelligence, that should be an instinctive behavior. If it gets bonuses for a bunch of it piling on the same target, then it will pile on. Beyond that, I use the following broad strategies for monsters:
Mindless. Pick the nearest target, charge it, and maul it until either it's dead or you are. If multiple nearest targets, choose at random, or whichever looks tastiest. This level of tactics I usually reserve for zombies, giant insects, and so on.
Retaliatory. Pick whichever target is dishing out the most damage. This is what I often use for monsters that are not terribly bright and are either fighting solo or don't coordinate well. An ogre is likely to use this strategy--bash the one who's hurting you.
Wolfpack. Haven't actually used this much, but I plan to do more of it in 4E. As a group, pick whichever target looks weakest and take it down, while avoiding its stronger allies. Good for reasonably smart pack hunters, like wolves and gnolls.
Soldier. This monster understands the principles of D&D combat--concentrate firepower, kill spellcasters first, guard your own casters, don't go toe-to-toe with the raging barbarian, avoid letting sneaky-looking people flank you. At the same time, it isn't a tactical genius and doesn't come up with really clever plans. Appropriate for intelligent fighters experienced in organized warfare, like hobgoblins.
Boss Monster. This is where I pull out all the stops and play the monster as if I were running a PC, using every nasty tactic I can think of. Appropriate for intelligent boss monsters and other extra-smart foes.
I wonder if the 4E DMG will have some suggestions on this?