OK guys, this is how I see it. If someone can please explain in detail where I'm missing it, I would definitely appreciate it.
If your topping off with extra nutes, your setting the plants up for possible problems.
Even though your adding the nutes at a specific EC/ppm, you have no idea which, or how much of each element the plant had already used since plants take in different elements from your solution at different times.
So for an example the plants used lots of N, but much less P and K, and your EC/PPM has dropped significantly. Now if you top off with more nutes to get your EC/PPM back up to that magic number. Is it not likely that your building up the P and K to toxic levels. Your meter is not going to decipher which of the elements is high or low. It's going to give you a total EC count.
Salts from the waste also build up in the solution as nutes are used in addition to bacteria and various pathogens. I would think that It's more work dealing with issues that arise as a result of short cuts than it is to just follow a recommended process of changing out the reservoir every one to to weeks.