When using reverb on a send, whether you make the send post or pre fader depends on what you want to achieve. In the following examples, the reverb on the aux track is set to 100% wet (as it always should be on a send).
Post Fader: This is the most common way of doing it. When you have a post fader send, the send amount is proportional to your fader level. So when you move the fader down, the amount of signal sent to the reverb comes down with it and vice versa. This is good if you always want the wet/dry ratio of your reverb to remain the same regardless of where your track fader is. The downside is that you're limited as to how wet you can get that reverb to sound - you can never get it 100% wet.
Pre Fader: When you have a pre fader send, the send amount is
absolute, i.e. independent of the fader level. So your send knob determines a fixed amount of signal sent to the reverb. Now you can configure a 100% wet reverb sound - by bringing the fader all the way down, all you hear is the 100% wet reverb coming from the send, and the send knob determines the volume of it. The drawback is that if you want to maintain a constant relationship between wet and dry with a pre fader send, you have to readjust the send level every time you move the track fader. But you can achieve some very nice effects with a pre fader send - for instance, if you automate your track fader to fade out the track volume, the wet reverb signal stays constant, and this creates the illusion of the sound moving into the distance.