Our FCTM used to have the on/off mix in it but it seems to have gone. It was overridden by the FCOM anyway in that respect.
I’m an advocate of all on or all off for the reasons others have posted. Yes, as soon as you get a mode change from TOGA on the GA, the flight directors will disappear but guess what? They come back on if you flick the switches! Amazing! What will they think of next...?
Also, more seriously, it depends on what guidance you require to maintain the desired flightpath. Are you visual, using basic (raw) data or requiring lateral and/or vertical guidance that can only be provided by the APFD/FMC? If it’s the latter, then you need to follow an appropriately programmed FD, especially with a specified RNP...