I know that we allow the wand to hit Mario to save frames but I've just tested it and found that if you have a P-wing and you meet the wand at the top of the screen, the scene will fade to the throne room faster. You'll save about 3 seconds. If you ever have an extra P-wing, from world 4 or 5 perhaps, that you don't use in 6/7, then it could be worth it to spend on a boss. Gotta save those 3 seconds.

It's well known and used in the TAS (with wall jumps). Using a P-Wing wouldn't be worth it for RTA because you don't want to sacrifice Fire Mario, and also because there's bigger time saves elsewhere with P-Wings.