If you keep stretched and dried mink pelts for several months before selling should you wipe the grease which migrates to the surface of the leather with a clean dry rag periodically? Or?

If you have that problem with grease then that hide wasn't completely fleshed. The few mink I did this year are bone dry and grease free once thy came off the boards.
But to answer your question yes you should wipe them down.

Also you could freeze them

I agree.

Mink pelts will turn an ugly yellow color once you freeze them.

Better to keep them in a dark, cold storage area of around
45*F to 55*F but not cold enough to freeze them,
in a place safe from heat, mice, & dermestid beetles.

Heat, oxidation, and damage by vermin cause the
most damage to mink pelt fur and leather during storage

w

Mink pelts that are finished are tough to store, they will yellow. The only way I've been able to have them store well is freeze the whole animal. Thaw out and put up they look like fresh caught.