My 29XX blisseys decay pretty quickly. By the time I'll get 20, or so, coins for them, they are pretty easy pickings. If I powered them up a bit more, I think it wouldn't make much difference.
I've heard that mon under 2K cp have a slower decay rate, but I haven't paid that much attention to the progress of lower cp mon that I put in gyms. I'm just counting on someone else's blissey to protect them long enough I'll get a few coins.