We feed grain free all the time. We switch the flavors periodically, but comparing a dog eating the same thing to us eating the same thing daily is silly. Look at the ingredients...there's usually multiple meat sources, as well as veggies and fruits in the food (at least high quality foods, anyway). I know I don't eat buffalo, chicken and salmon at the same meal, but dogs do, so mixing up the foods isn't comparable to us eating the same thing day in and day out.
As far as switching foods up causing diarrhea, that's most likely to happen if you make drastic switches, like if we took our dogs from the high protein grain free diet they are on now to some cheap shit full of grains, with low protein and lots of fillers.
We've gone as far as switching from Before Grain to Natural Balance Alpha:
but since they have similar protein levels and have no grains, we never have a diarrhea problem. We've also fed Taste of the Wild (also grain free).
But with any brand, we do like to rotate the flavors. It's not really a big deal to the dogs, but it makes us feel better.
And for the record, my wife is a hobby breeder, and we've done a lot of research on dog's and puppies' diets, so I'm not talking out of my ass here.
Say hello to Diana, a five week old Pomimo (American Eskimo/Pomeranian hybrid) from our current litter.
