Ro water question

Do I need to use a ph pen or ppm, ec pen

Both but pH is real important I found out the hard way

I don't use either.
For the pH, I use the drops.........and I've never owned a ppm or ec meter.

Kinda depends on how your growing.
Not absolutely necessary but they're nice to have.
I use drops for PH because they're cheap, available everywhere, they work, and no calibration or storage fluid needed. I do have a nice PH pen but <shrug>.
EC pen is very helpful growing with inert mediums and hydro. Soil/organics, not so much.