All the gamers I know have different rules on walkthroughs. I have one friend who takes it to the extreme and outright refuses to even glance at a walkthrough while playing a game. My other friend will look at a walkthrough, if he's having a lot of trouble with a certain part of the game. I have yet another friend who will play through a game using a walkthrough, so as to not miss any secrets.
I'm somewhere in the middle. I usually never need to use a walkthrough. The toughest challenges are the most fun when you find your own way of overcoming it. However, sometimes, I think it's unavoidable. In Fire Emblem: Path of Radiance and Radiant Dawn, there are invisible items hidden on specific squares on the map and it's impossible to know where they are without a guide. So for that, I'll use a walkthrough, but I never, ever, read the strategies in the walkthrough. Path of Radiance could be beaten in about 10 hours, but it took me around 20 because I had to repeat a couple levels at least 10 times.
For RPGs, I'll read a walkthrough after completing a dungeon to see if I missed anything. Not that it matters, because if I did, I don't return to get it, anyway. I won't look at walkthroughs even for optional bosses, though.
So how often do you use, if at all, walkthroughs?