* Disclaimer: I am not good a keeping up with the water testing, so I might be exception in the group.
I do not ever test my DT for amonia and nitrite. I do test the QT after I start it and before adding livestock to it.
I also do not test much the pH. I usually test it once a year or so. The fact that I am dosing Kalk and not 2-parts & the fact that I am running a sump with the light on the opposite time than DT light, makes me more confident that my tank's pH is stable. And in 9 years since I started my first reef I never had a problem that I could tie it to pH. For a few years I was using a pH probe on my ReefKeeper, I even replaced it a couple times at the recommended interval. It didn't register any pH problems.
The only time I kept a really close eye on pH was during a QT hyposalinity treatment to get rid of ich.
But it depends for each tank: the light cycle, if you have a sump or not, if you run a skimmer non-stop, etc. Here is a good article on pH:
http://www.reefkeeping.com/issues/2007-05/rhf/
As for nitrates I would not let them go over 20 ppm if the tank has fish & starter/soft corals and I would not let them go over 10 ppm if the tank has SPS. I should also mention what I noticed & what other folks stated in articles or forums: a steep variation of the water parameters it is most times damaging to the livestock even if it is a variation towards the ideal value. So, for example, I would be careful not to do a 50+% WC to reduce the nitrates from 40 to 20 in one shot. Beside a steep variation of nitrates, the 50% WC could greatly affect other parameters at the same time, thus having a way bigger impact.
Have you thought of adding a medium that would consume the nitrates such as macro-algae, mangroves, or anaerobic medium (there is some synthetic porous ceramic that is promoted as a good media for bio-filter on surface and anaerobic bacteria for nitrate reduction inside)?
