i dont. i just make sure salinity and temp are in the same ball park. the reason for the water changes, from my understanding, is to help replenish some of the trace elements as well as remove dirty water.
Nope. I get the salinity in the range where it's supposed to be. I throw a heater in there when the RODI is almost complete. I don't bother taking temp readings since it'll be close enough with a heater in there for a couple hrs. Whatever the differential in temp/salinity is won't have a drastic effect on my tank if I'm only doing a 20% water change.
It depends on what your reason for doing water changes is. If you are trying to keep the tank stable or you want to maintain a certain level of ca/mg in the tank, yes you should test the water and raise the levels if you need to.
Per Joe it depends. If for some reason you pumped up your Calcium to high you would not want to pump up the new water. If on the other hand it was to low you may want to. Then there is the issue to just add the wanted sup if low after the WC.