July / August best time for the Bugaboos. June or September are probably rather hit and miss depending on the year.
Check out the condition reports at http://www.acmg.ca/mcr/archives.asp for the last couple of years to get feel for things.
You can sneak around a bit in the shoulder season either side. There are lots of non committing sport crags in the rockies around Canmore and Banff for when the weather cr*ps out. In general, these are fairly forgettable but more fun than you would think.
I concur about the rockies alpine guide being a book of lies. But it makes for a welcome change from the uber high intensity beta photo mega topo type guides.
Capitol Reef sounds good. It is going on the list.
Tetons are already on the list !