It really depends on the character, for me. Let's see, of the 3.0/3.5 character's I've played enough to have developed a good feel for them, I've got one with 4 base and 4 prestige classes, one with 1 and 3, one with 1 and 1, two with 1 and 2, one with 2 and 1, one with 2 and 0, three with 1 and 0 (one of those was constrained by rules, however, or would have been in the 1 and 1 category), and one with 2 and 2. I just build them to what they'd want to be able to do, and what I want them to be able to accomplish mechanically. I don't think there's a right or wrong answer to the question of whether or not to multiclass, though I've tended to favor it.
Just do what seems to be the right thing for you, your character and your gaming group. The last bit isn't trivial. None of us play D&D in a vacuum, we play with other people, and their expectations play into the choices we're going to make. If your group frowns on multiclassing, take that into consideration. If they expect every character to be able to sneak around, maybe you do want to multiclass into something that lets you get Hide and Move Silently as class skills. If they love trying out all the new rules, try some of the new classes or pclasses.