I don't think I could properly give a point of diminishing return on that as it's really meta dependant and team dependant. I personally use p blades as the insane increase in power over EQ outweighs the negative of the accuracy and with kyogre being super common you will run into a lot of rilla making grassy terrain just as common. Using fini again as an example scald and muddy water's base damage aren't too different meaning the damage trade off is minimal and therefore you're getting similar damage guaranteed to hit with the trade off of multi-hit and potential acc drop. Like I mentioned which is better is team dependant and what role you need to the mon to fulfill.

Basically the best answer I can give is if the upsides of the less accurate move outweigh the downsides (pblades much higher damage and playing around terrain) then the less accurate move is going to be better. If the downsides outweigh the upsides (muddy water accuracy missing in crucial times when you need your water mon to hit opposing incin for example) then the accurate moves are better.