There are many other toxins that build up in the water besides nitrates. Doing WC's based on a tanks nitrate reading is not a good idea IMO. Awhile back AquaChem posted this in a discussion about doing WC's:
"How confident are you that nitrogenous waste, phosphates, and GH / KH are the only thing that builds up? Don't make the mistake of confusing undetectable/unmeasureable with not there.
Fish don't just excrete ammonia. They excrete many, many organic molecules (metabolites, proteins, etc) that could potentially be toxic. These compounds could have a number of potentially hazardous effects on a tank, such as (and completely made up on the spot): noxious to fish gills, bioaccumulation, or fueling dangerous (or unsightly) heterotrophic bacteria or algae species like cyanobacteria."
You also stated you always have high nitrates so have you tested your tap water to see if there are any nitrates present? What type filters do you have and how often do you clean them?
I've been in the hobby since the 70's and do 50% WC's in all my tanks regardless of tank size or bio-load. Since I have tanks up to 220g I use an Aqueon Water Changer which makes my WC's much easier.