That is incorrect installation of a roof and voids the warranty.
Well the insurance woman I spoke to today didn't make it seem like it was..In fact she said we were lucky up here that we are allowed to do it, because they were not allowed to in FL. I do know of others who have done it because I have seen it done all around here....I will find out soon enough.
I myself never had one done, but there are (or at least were) many legit company's that do it here in FL. It depends on the shape of the shingles that you are covering.