Well any animal bite is an open venue for infection but as long as your rats are kept in a clean environment and they are in themselves healthy I wouldn't worry unduly. Just clean the area well with hot soap and water, apply some topical triple antibiotic cream (neosporin, polysporin type thing) and keep it clean. I've been bit here and there and haven't had a problem as long as I did that.
Remember momma rats, even nice tempered ones, can be darn mean when they have litters so mind your fingers when tending their enclosures.
