It depends.
On something like inFamous, I'll play twice. Once straight evil, the other straight good. And I normally do evil first to get it out of the way. In most games that have a binary system, the evil path tends to be the mustache twirling, derpy kind of evil. Would you like to drop kick this kitten or heal this puppy? Before you choose, keep in mind that the outcome is probably cosmetic and the good choice requires zero gameplay sacrifice! Blah.
In something like Skyrim, I tend to ignore morality completely and only focus on what I think is fun. If I decide to do a role play intensive run, I'll exercise ridiculous mental gymnastics to justify it. "Sure I'm a GOOD person... but if I don't take over the Dark Brotherhood, someone who's REALLY bad might do it!"
I wish more games had quests like Tenpenny Tower from Fallout 3, or were more like The Witcher, which really should be called "The Witcher: A Series of Bad Choices." OR, I wish that one of the paths in the more binary games required actual sacrifice. Something like Bioshock seems like it would do this, but you can max out everything on the good path; there's no point in harvesting Little Sisters other than being a dick / seeing if it affects the ending, which isn't great.
@kriswrightNot to mention... Vader in the OT isn't mind numbingly stupid, impulsive, and rash. Granted he's got almost two decades to cool down between Ep III and Ep IV, but nothing of what we see in Vader in the OT suggests that he'd be so dumb as to just accept the fact that the Emperor doesn't know how to save Padme despite that being the main reason he turned.