Where possible and when appropriate, the back exit is often the best to use.
Not only is it less crowded to take the patron out via the back exit, it also stops other customers from seeing the problems taking place and potentially put off those waiting in line from entering.
A good door supervisor will be able to look around the bar and be able to judge approximately how close to full capacity it actually is. Furthermore, in large clubs the establishment should be recording just how many patrons are actually inside.
Knowing how full the bar is, will have an impact on the decisions you make. It can also impede your ability to do your job. For instance a busy bar will make it more difficult to remove any trouble from the premises.
It will also be more difficult to keep your colleagues in direct line of site. Although all door supervisors should be able to communicate via radio.