I requalified at Boston this year. I didn't run any other marathons to get a better time. Times were overall slower by 7-10 minutes due to the heat, and for many people much more like 15-30 minutes.
It *does* bother me that I'lll be running in the same corral as people who ran a net downhill marathon, because we will not be running at the same pace, creating roadblocks of slower runners as the race goes on. This is what happened this year as well. Everyone is running too quickly at the beginning, and then they all burn out after the half way point and I have to weave around everyone as they walk the Newton Hills.
I ran with 10 other people who had posted a similar qualifying time but at downhill races, and I beat them all by 10-30 minutes. And guess what, for the next Boston, they will be starting in the same corral again. Ugh.
While it is not technically cheating to run an easier course, it is silly to count a downhill marathon as your PR.