By far the most popular time to climb munros  are in the months from May to September as the snow will more than likely have melted in these months. However as previously mentioned always be aware of the weather as it is always changing and nothing should be taken for granted. Even though these are the most popular months on some lesser treaded hills you can still have the hill to your self, and some of the popular ones can be busy. Expect to queue to Bagg the Inaccessible Pinnacle (Sgurr Dearg)>/a> on a nice spring /summers day. 
Winter walking in Scotland is another level al together and should only be attempted by experienced climbers. There are many winter training courses available to take during the winter months and should do some of these before venturing any winter munros. The Winter Munro's in Scotland contain some severe ice climbs and there are many deaths every year in avalanches and falls. Even on a good day in snowy conditions the mountains will look so close it can be dark at 4:00pm.