In reply to Pietrach:
It can snow any / every month of the year on top of Ben Nevis.
The main thing that will prevent you from camping on top of a Munro is high winds although they often are OK late may.
I think if you have a sleeping bag that will definitely handle 0 degrees C sleeping naked then you can wear some clothes if need be to increase warmth and you should be fine. Also bear in mind if its a clear night its normally colder in the glens than on the summits as cold air drops.
I think the likely temperature range you'll find at munro level is -2 to 15 degrees. At car park level it could be super warm (today is over 20 deg at many car parks) however it could easily be 10 -15 degrees colder than that. Currently at munro level on the west coast its 15 degrees C!
Post edited at 12:14