I don't concern myself with prospects in re-draft leagues. It's rare to pick up a Buster Posey-ish prospect and have him produce right out of the gate and sustain it.
Keeper leagues are a whole different story. You can rush to pick up the guy(s) to have on your team for the future or use the hype surrounding said prospect(s) as a bargaining chip to land someone you want on your team.
As for my keeper league, I'm already picking them up just as their stock is beginning to rise. I scour the minor league system and put them on my scout list. Last year, I drafted Eric Hosmer in our reserve draft and just a couple weeks ago, I picked up Shelby Miller. Most everyone in my 12-team league said, "Shelby who?" I like to beat everyone to the punch before they know the prospects. I have a pretty good track record when it comes to minor leaguers making it big in the majors. I think this helps me field a competitive team year in and year out and keeps some of my friends a notch below me.
Of course, the FBC helps me keep one step ahead as well.