Yeah, somewhat, you can check it to give you an idea what's going on. For example, if you pour in 200 PPM, and it comes out 1500 PPM, you have alot of crap in your soil. Back when I was anal about checking everything,lol I'd flush my medium when it had alot of buildup, and keep flushing until there was only about 100 PPM of waste picked up, as I poured my 35 PPM water through. Then, I'd be pretty much starting with a clean medium, giving them a fresh start.
Now, that I am used to feeding them, I know not to get carried away, and don't even check runoff, or soil PH anymore. I like to top-dress my soil with lime, at around week 6, giving me more peace of mind about the PH. Idk if it's even necessary, I doubt it, but I like to do it anyway, just in case.