Yes, as a comic strip artist, I highly recommend learning to draw upside down pictures, for the exact reason you stated.
Interestingly enough, this applies more so to forgery. Many forgers practice signatures upside down, because the way we write letters is so a part of us that if we try to forge a different handwriting, it'll still have some of our own handwriting in it. So a forger learns it upside down, because the curves and loops are no longer "letters" or anything internalized, but something completely different, bypassing the logic part of the brain and getting a better forge (but don't forge signatures
)
-Ramon http://razor.ramon.com