I’ve been experimenting with this lately because I was tired of my free-floating sections moving around on the slides when I added coverslip medium (Vectashield) and applied the slip. I found that letting the slides mostly dry after mounting freefloating spinal cord sections helped them stay in place better when coverslipping. However, I keep a close eye on the slides so they don’t get bone dry - just until most of the PBS is gone. I would probably never leave them drying overnight after IHC - maybe this would in fact be okay but I’m not courageous enough to try (I have also heard that overdrying increases background.) I have not yet had an issue where the dryness of the sections interfered with the quality of IHC staining.
For thinner sections that are directly mounted onto the slide at the time of sectioning (therefore all IHC steps are processed on the slide), I will let these dry a few minutes before medium + coverslip, but I let them still be quite wet with PBS.
If you are using the hardset mounting medium from Vectashield I think extra PBS on the slide may interfere with its ability to harden, so if this is the case then you may want to wait until the PBS is mostly gone before coverslipping. However I use the softset Vectashield on somewhat wet slides and just seal the whole thing up with nail polish and it seems to work fine.