Chins are quite capable of chomping good if they wanted.
They only bite as hard as is required to get their point across.
Grooming is a gentle nibbling that tickles and give you goose bumps. Don't be afraid to put your face up close to the face of your chins.
If they want out and your hand is in the way they'll first try pushing with their own. If that doesn't work they'll take your hand/fingers in their teeth and try to move you. Not hard, no pain, just get out of my way.
If you were trying to get meds into them or anything they really hated, they'd bite you hard enough to make you jump and say a ouch. They could keep up this annoying and somewhat painfull attack all during the process. Though only getting slightly harder, not bringing blood but definitely leaving a mark.
Now if 2 chins were in attack mode and you reached into grab one you could end up with a nasty slice and blood.
Would they ever go savage and try to tear the face off you like a squirrel? I doubt it, can't see it happening.
So see they bite as hard as required and no more, so don't pee them off