drainage water

No, never

it's good to know a baseline and can help diagnose when things go wrong (without a baseline you can't assess that as well). i've never tested my runoff, lol. So, it's not essential. If you consistently grow healthy plants with minimal adjustment you can make with simpler obsevations, you won't be testing runoff much, if ever. I keep a 100-pack of phstrips that resovle to 0.5. They are multiple years old at this point, but good to have on hand. TDS pens will give you different ppm readings based on the brand/region of origin. EC is the measurement you want to use if comparing different gardens to avoid the potential apples to oranges mistake. I religiously track the ppms of what i feed (calculated from guaranteed analysis labels). correlates to why i have few poblems at this point no matter how many strains are pulling off same reservoir / mix of fertilizers without poblems of deficiencies nor toxicities. definitely sped up the learning curve of feeding.

Yes you should it tells you what is going on in your media .I do weekly distilled water test too

Be more concerned with what is going into the soil rather then out. The drainage should be checked if you suspect deficiencies, your current neut mix is "ph perfect" so long as it's R.O or you ph your water beforehand it shouldn't be a concern. Hope this helps!

That's often done to figure out if something is going wrong in the soil.... but I don't do it as a general rule - more concerned with what I'm putting INTO the soil... Cheers!