I thought this question might be interesting to ponder.
For me, I think it depends.
Sometimes I find my favorites irresistible and run them a lot; othertimes I feel it's best to only run my favorites on special occasions, such as when I have visitors, so as to minimize the possibility of them developing mechanical problems.
However, I always want to run my favorites a minimal amount to keep them in good shape. I know it's rarely good for any train to hardly ever be run.
I can expound more about this, including mentioning particular models, but let's hear from a few of you folks first.