I think it should be strongly advised, but even with a full paintball mask on, you can easily get face injuries. The slits in the paintball masks for ventilation are just smaller than the size of a BB, but with the force that the BB is travelling at (especially at close ranges) it's enough to go through, and then you get some injuries to the face.
However, even though BB's may still get through, they will be drastically slowed down by the face mask. For CQB, I'd say it's a really good idea, but not necessarily mandatory because face masks do get in the way with certain types of AEG's, and in the end it's all up the the player if they want that added level of protection. Provided they have proper goggles on, you've done your job.
This though, is just my humble opinion, take it for what it's worth.