Temp isn't going to hurt you near as much as everyone thinks. Even if it was to shorten the life of your CPU 50% you are still talking 5+ years of life, even for people that don't upgrade very often 5 years is a long time. I ran it for around 3 years on an Athlon 1Ghz Thunderbird and the cpu was still working fine when I retired the system due to motherboard problems a couple months ago.
Several of us were using SETI, on the techspot team, a while back before they switched to the BIONC or whatever version. Then a bunch of us quit because BIONC sucks.
You don't really have a choice on how hard you push your cpu when you are running any of those projects, they use unused cpu cycles, so it uses everything the rest of your computer isn't.
If you are going to run it I recommend you run it all the time (except for maybe when you want to game, but even then it should be smart enough to let the game take the cycles) simply to avoid many heating and cooling cycles. A continous hot is better than cycling hot and cool.