Particularly with regards to telling myself I'll play through again as evil, and then not doing it. For my first playthrough though, I do my best to do what I feel I would really do in that situation. This leads to an overall good approach, with occasional petty evil deeds when someone really irritates me. I never choose the evil action just because it benefits me more (better rewards, whatever) but if some side character is all cocky and snide and making a general nuisance of himself, there's a good chance I'll screw him over.