When deciding on what tactics to use for an opponent, there are many factors to take into consideration. First and foremost, what are the creature's motives in the encounter? If it's merely survival, that will dictate what you do with the creature. If it has friends and allies it can rely upon for aid, that also changes your tactics. If it has a different motivation, such as to prevent the characters from accessing a site or an item, that will also change your tactics.
But regardless of the creature's INT or WIS scores (both of which dictate what we would call sentience), a creature with any degree of intellect has motivation, so use that as a base. Then, apply INT and/or WIS scores to the equation. Even creatures with a low INT score but decent WIS score can use some tactics, such as making sure it doesn't get flanked (or getting out of a flank), or using its special attacks to best effect.
Probably the best way to gauge tactics for creatures is to decide who they attack, and who they will stay away from. A 'dumb' creature should attack randomly, while a cunning or smart creature will make more informed decisions about its actions in combat, such as attacking the preceived 'weaker' target so as to reduce its number of opponents. By the same token, a creature who is essentially clueless about how things work (ie, a low Wis score) should not intentionally take advantage of situational modifiers in combat - in other words, it might not take that 5' step to gain a flank with its ally against a foe, or might make a move without regard to possible opportunity attacks. This is not to say that the creatures loses these benefits where applicable - they just don't know well enough to make them happen on purpose.
If you need a rule of thumb for this situation, I would say any creature with a 5 or less INT score should roll randomly to decide who it attacks, and any creature with a WIS of 5 or less should not use situational modifiers (such as flanking bonuses, higher ground, opportunity attacks, etc) to decide its tactics.