Trying to establish whether it's appropriate when using planers, circ saws etc. Anyone able to confirm whether it's just general good practice or entirely down to risk assessment?

General good practice cannot be in place without risk assessment so look at each case on its own merits

I work for a large manufacturer of bedroom furniture we always use the RA approach to establishing if PPE is required but I have to say the answer to wearing safety glasses or goggles when using fixed and portable woodworking machinery is usually yes as there is always the potential not just for wood dust and particles but also for tooling to break.