Depends on which game I'm playing in. I really dislike PrCs that need a bunch of feats, some which aren't things you'd pick up anyway. Because if in character it turns out that I want to go down that road, if I need to pick up two feats I'm 4-6 levels away from fulfilling prerequisites until I can start. And that's a long time.
(Mind you, the need for such "sub optimal feats" for balance is a different story. I'm talking just about the effect it has in making me want ot plan ahead for my character.)
Heck, I remember taking ranger because a PrC I would like *IF* I happened to find the secret organization it was part of and get invited to join, because it required Favored Enemy. How silly. (Harper Paragon in the FR.)
So in one game where advancement is fairly quick and we'll probably play to mid-high or even epic, I do work it out. Now, I don't take "Level 20" as some holy grail - maybe I stat out until 16th (my current character), maybe I think through near epic.
In other games, I just play it by ear. Whatever makes sense for the character at the time. It ends up being a lot more organic, and usually less powerful. Still very fun, just a different focus.
Cheers,
=Blue(23)