It depends on how you define the word 'need'.
I've done a few bubbler grows without an ec meter. nothing died and the yields were decent but the plants looked like shit, any colur you want except green.

For my passive hydro grow I don't check the pH but I have measured it in the past and as I use ro water, everything is the same so the pH now is the same as the first time I mixed up a res to that recipe.