Over the years, I have always maintained the same shutter speed whilst filming, with my iris and ISO being my two variables. With 25p and 50i, this has always been 1/50th or occasionally 1/25 for low light. With 50p, I am not sure if this remains the case. I have recently been shooting 50p at a constant shutter speed of 1/50th and no real problems there, I haven't tried it with 1/25th though. There is a school of thought that says we should shoot 50p at 1/100th which obviously reduces the light coming in (good for outside but bad for inside). I can see how this might be of benefit for slow motion but is it rally necessary for general use? Any thoughts?

The general guideline is for shutter speed to be 2x the shutter rate. Looked at in the opposite way, the shutter is open for half of the time between shutter activations.

Without going into the history and things that drove it, think about the motion blur esthetic you want. Shutter speed (the time the shutter is open) determines how much motion blur you have. If you want stuttering crisp frames, by all means shoot at 1/100. If you want the normal blur of 24FPS film, then set it at 1/48 or 1/50.

There are sound reasons for 1/48th one of which is that was deemed as simulating the human eye.