It really depends on the game for me. For the most part, I always check PS3Trophies to see the road map for what what kind of experience it will be and how doable it seems. Like right now I'm playing through LEGO Marvel Super Heroes, and I know what to expect from it so I know I can get the Platinum. but then there's other games like any of the GTA or Arkham games where I know that trying to get the Platinum is going to take a ton of hours and will be a mostly frustrating experience, so I don't even bother. I try to get as many trophies as I can, but I try not to spend too much time on anything that might involve needing the help of others like in MP or is extremely luck based.
Assassin's Creed: Brotherhood is the prime example of a game I hate because I just need one trophy for the Platinum but I don't have any friends who would help me and I don't want to bother and strangers to help me boost for it, so I just leave it alone.