I'm sure it's been suggested before but unless I'm missing something already in the game I can't find any way of doing this...
It would be nice to have a toggle similar to the radar range one for the Geoscape that shows the SAM site and base defense intercept ranges, which would aid in planning out where to put SAM sites etc. It is possible to do that now but you have to wait for a UFO to show up on radar before the coverage appears, which isn't ideal.