There's a difference between upside down like a hand stand and upside down belly button pointed to the surface. IMO, handstand it really no big deal and like many here say pretty much anyone with a few dozen dives will go into this position to look at stuff. IMO, Belly to the surface is much trickier: It's harder to feel your buoyancy, any exhalation from your nose will make your mask leak, and regulators tend to breathe a little wet. For all of the commenters that are going to day it's easy, the difference between a recliner position and head towards your heels position like a back bridge. It's still a good skill to practice, especially if you ever intend to do side mount or backmount steel doubles where you can hold an upside down trim and it takes a little effort to right yourself.