I consider them cheating in Multiplayer. It's all fair game in Single, though.
Cheap tactics can be quite entertaining against the AI, too. Diablo II's Andariel, for instance.
Necromancer casts Iron Maiden (a percentage of damage the target deals is reflected) on Andariel. Necromancer surrounds Andariel with Bone Walls. Andariel kills herself attacking the bone walls.
Priceless.