Does it lead to a full blown fight. Not really. The side butting is normal when showing dominance. Usually the other one will submit unless you have two dominant rats. But if you only have a two or three, they will settle their differences without a full blown fight. Even if they do go into a rat ball, it doesn't necessarily mean they hate each other. They are working things out. If you do see a bite which to be honest, is not common, then it's time to consider neutering the bully.