Mmmm. The issue with shifting F/R simultaneously is that the chain can go quite slack when going to a smaller ring and smaller cog which increases the potential of dropping. When shifting big ring to small, it's best to keep up a small amount of pedal force so that the top run doesn't go slack and and get whipped past the small ring by the spring force of the FD. In my experience, this seems to be more of an issue with compact cranks and triples, where there can be more distance between the DR cage and the small ring.