If you look at the Paddleducks site there was a lengthy discussion on this not so long ago which also continued on here as well.
Basically what do you want it to tell you? If you are interested in the pressure of the tank as well as when it is empty then you want it before the regulator but if you want to ensure that the gas pressure at the burner is remaining constant and therefore that the regulator is doing it's job then you want it after the regulator. There will be some who advocate both!!
My gauge is on the tank itself as my main concern is whether the cooling effect due to evaporation is becoming excessive and affecting the tank pressure. If this was to drop off enough and become lower than the set point of the regulator then it would also affect the burner pressure.
As for the range, that was where the lengthy discussions came into it but these concerns would only be relevent if the gauge was on the tank.