Inlet pipes for mixer shower or wall mounted taps

[pcsparky]

I'd like to install a thermostatic mixer shower and possibly wall mounted bath taps. Should I purchase these first so that I can read the instructions in full or should I be able to install the inlet pipes beforehand at 150mm spacing?

If yes to the latter should I leave a good length of pipe sticking out and cut this later or should it be cut to a certain length beforehand?

 

[swbjackson]

They always used to be 152 spacing but there are quite a few now that aren't so I would play safe and get the shower and taps first.

Mike