But even if the little bit of yeast, 5-20 grams, did go into solution, would that really make much difference in your reading? I can see adding a half quart or yeast. But as the poster above said, they don't dissolve, so maybe they have zero affect on the reading.
I am sure if I forgot and took my reading the next day there would be a difference as the yeast have been consuming sugar.
Anyway, no big deal, I just get so mad at myself when I forget to take a reading before adding yeast... an old habit from the canned kits days where I never used a hydrometer, cause the OG should be the same every time.