I had my hand slapped by tech support for that when it was DNIC integration I don't know about IP.
lol... if I had a nickel for every time tech support tried to slap my hand and said don't do that or it will never work, yet it worked fine... well, lets just say I would probably have enough nickels for the pop machine, but not enough to pay for lunch with them and tick off the waitress at TGI Fridays. Still, that's a fair amount of nickels...
Now, I wouldn't put a BLF key on 15 phones for each of the VM ports, but if it is just for 1 phone for you to monitor, probably not a big deal.