Google seems to use what I'll call "one-way" stemming. For instance, you can have ~keyword1 show keyword2 in the results, but ~keyword2 won't show keyword1.
So if I have my website optimized for keyword1, which stems into keyword2, then does this help me if people are searching for keyword2? Or does it not help me because keyword2 doesn't stem into keyword1? My guess it that it doesn't help me, as my SERPs are really bad for keyword2.
Note: I know that the Google knowledge base says that Google doesn't do stemming. But perhaps this is wrong, because when I search for a certain two-word term (politically related) it comes up with results for another two word term in bold.