My cat is 13 years old and we've been doing that since he was a kitten. The vet showed us a neat trick when he was a baby that was basically holding him under the armpits, and he's never had an issue the vet has picked up. Pun intended.

Now that's one cat and one story, but I don't think it's a big deal. You're holding around the ribcage (which is very strong) and they're not unsupported long normally before you put them to shoulder (or head, as cats are wont to do themselves).