Is it safe to use charger for macbook with a 15" macbook pro?

"It is quite safe. I have used my laptop (HP) with underrated power supply for almost two months (waiting for replacement part) and there are only two issues with it:

Your battery will take longer to charge.
From time to time, when you do something that needs more power (e.g. watching video), it will completely stop charging battery. Once your power requirements go down, charging will resume.

If you keep battery always topped, I see no problem with that arrangement."