In reply to wilfbiffherb: I think the SMC Munro book assumes you'll be out in all seasons and mentions where winter conditions make a difference to summer ascents. Other guides prob. do the same.
The difficulty and joy of winter is that it encompasses an immense sliding scale of conditions. eg.
-really cold but little snow - like summer but maybe slippy on rocks or with ice
-relatively cool with lots of fresh deep snow - possibly a huge slow, slog where crampons and axes will not help (skis or snowshoes might), avalanche issues
-cold with a depth of hard settled snow - easier than summer as it smooths out the bumps but slippy where steep (crampon/axe territory)
-warming temps with rain on a hard old surface which will sometimes hold your weight, sometimes not, and which sits on a bottomless layer of loose sugary graupel
etc, etc and you can get different conditions on the same day on different parts of the same mountain due to effects of sun, wind, recent weather history, temp, altitude, aspect. Winter climbing grades can be argued up and down depending on conditions and the same goes for the difficulty of walking routes.
Understanding what you're facing should inform your response: crampons on or off? vary the route? avoid avalanche terrain traps? high, icy, windy, exposed, snow free, ridge or safe but slow and tiring trudge through deep drifts in the valley? sack it all and go to the climbing wall/pub?