"...we were thinking of the last three weeks in Sept...The other alternative is the end of May beginning of June."
I live just South of Vancouver, B.C., and therefore can state from experience you OUGHT to be 'safe' weather-wise anywhere in the Pacific Northwest within either of those time frames. But, July/August is unquestionably the best time of year for the warmest, driest weather...as is the case anywhere along the 49th parallel. Obviously, the further away one gets from those two months, the cooler the temps. Pack accordingly! Crowd-wise, it would depend on where you might want to be and what you want to do.
You might want to Google the average temps/weather conditions for any given locale in any given month you're planning to be there. That should aid you in both the traveling schedule and 'packing' departments. ;-)