Chasing payment from customers can be a tricky business. You don’t want to offend them yet at the same time you also need to be firm (and maybe a bit scary too). There really is no easy rule of thumb in this matter. The more you know your customers the easier it is to approach them. Obviously some of them are impossible to deal with so just get the hammer down and get it all done. Some others probably just “missed” it.
It can also very well down to the intention itself. Are you trying to show “I am the boss?” or are you trying to admit “They’re the boss?”. It is off course important to have a good balance between the two. Over emphasizing one over the other will result in either one being the “boss” which we don’t want to happen.
Cocky and scary, or firm but fair?