Well, since the aquarium just started cycling, it's actually Ammonia that will spike first. Then Nitrites, and last Nitrates. I would monitor them all to make sure that the cycle actually starts and then to make sure that it is over. Better safe than sorry.
Once the aquarium is done cycling, I'd only test for nitrates (and ammonia and nitrites) on an as-needed basis. I don't test for those three parameters unless I see something that doesn't look well in my tank...