I sometimes there is more risk to the blowee than to the blower. (Why is it called "blow job" anyway, since it's actually sucking?) I've never been unlucky enough to encounter the dreaded "biter", but I've had a number of guys who were not careful about how they used their teeth, or have been simply too rough with their mouth around the head of my cock. One gets the impression that some of these guys don't have a dick of their own, or if they do they forget about what the experience is supposed to feel like for the receiver, they are just intent on gobbling you up and getting the job done. You CAN use the teeth, contrary to popular opinion, but you have to be very gentle and be aware of what you're doing, think of just stroking the penis with your teeth as you would with a feather.
If someone sucking your dick is treating it too roughly, don't be afraid to tell him so. Very often they don't realise, they will be apologetic and will proceed to do it properly. On rare occasions I have just had to withdraw, giving the somewhat ambiguous explanation "This is not working for me."