Bluestone has specials most of the time. Right now tall the selected specials are are all 50% off. The problem with waiting for the big sale is that some things are gone. Not many,but some.
High Country Gardens does not have the really big sale Bluestone does, but they also have good sales all along and end of season sales. I got some really big amaryllis bulbs 50% off in January for instance.
Both stores have e-mail newsletters and send out news about specials in them.