I've seen them at Glenshee twice from the roadside, both times in March when they come lower down the slopes.
Cairngorm is no longer easy as you can't just use the ski lift then wander around the plateau as you could in the past. Much more walking is involved to give yourself a chance of seeing them in the Cairngorms, in my experience, and I've seen nothing there on several occasions, despite putting in the effort.
This site is reported to be very good, and at relatively low altitude, though quite a distance from Aberdeen (Belach-na-Bah, on the way to Applecross ) see here:
http://www.wildaboutbritain.co.uk/forums/british-birds/17049-ptarmigan-west-coast-scotland.html
Steve