Just thought I'd chime in...

I've done both with my rats/afs (this can go for mice too): added new blood for breeding, and held back. My preference lies solidly in holding back and in/line breeding. Here's why:

Your colony will develop immunities within itself very quickly, and your animals will be healthy and strong. Bringing in new stock introduces new blood, true, but with it can come diseases, sicknesses, faulty genes, etc. If you have good strong genes in your colony, there's no reason why you should bring in more blood. The one time I did it, I regretted it sorely. I lost some of my best breeders due to God knows what, even though they weren't housed with the new guys.

Every now and then I get a spinner, maybe 1x every six months (I produce upwards of 100 rats per month, just counting fancies), but I just feed them off when I notice it and that's taken care of.

BTW: Spinning, if it happens to a rat/rodent before about 1.5 - 2 years of age, can be attributed to a neurological disorder, which is not harmful to your snakes or anyone else in the colony. If it develops later in life, it is likely a bacterial infection, and it can spread. I've never had it develop late, but if I did, I would have to euthanize that animal and the animals housed with it. I can't risk that going through my colony. This is a little tid bit I picked up off of a vet friend of mine when I asked her about it.