I like to see all the possible dialogue options in games, so I'll typically play through at least once per gender/race/class.
That still misses a bit, since I don't have time to play through as all possible combinations. But, the way I see it, getting one playthrough of each gender will get all of the important gender specific dialogue trees, and playing through as each class will get you all of the important class dialogue trees, so that covers most of it.
(Although, I do have a very specific pattern. In games like Dragon age, I go with a male mage, a female rogue, and a coin toss to decide the warrior. In games like Fallout, well... it doesn't really matter in that game, so I typically just play as a male.)