I haven't experienced that and I'm only an owner for 4 months now so nowhere near an expert but I would say it's fine as long as the stiffness isn't due to it still being frozen which by your method of defrosting can't be the case.

I actually take mine of of the freezer the morning of feeding, put in ziplock bag and just let them air defrost. By the time 8pm-ish comes around when I feed mine, the rat is good to go with a little warming with a hairdryer.