Thanks @fmoehring and @LegakisL. I usually stored at -20C and stuff looks good. But recently I also stored at 4C for well over a month, and the staining (IHC and ISH) actually looked great. Which is why I brought up the question. For tissue slices that haven't been stained, we do -20C and I know some people do -80C. But once mounted, I think I may go 4C.
I imagine there are cases where -20C or -80C would be superior. Perhaps for a labile antigen or fluorscent protein. GFP would benefit I think from the cold. Already native GFP signal is somewhat weak. But with antibody staining, I think 4C would be adequate.