Should I do water changes with OK readings?

You didn't mention nitrAtes. Those are the end product of the cycle and build up. Water changes help to remove them. As long as your tank is cycled you SHOULDN'T have ammonia or nitrites anyway; but you WILL have nitrAtes.

Regardless, even if you have NO nitrates (which is pretty darn near impossible - unless you've got an incredibly heavily planted tank), you still need to perform regular water changes. Ammonia/nitrites/nitrates, etc are not the only things present in the water. There are other elements that we do not test for. There are innumerable numbers of other organic wastes that are present that get "reset" with each water change. Heavy metals are one example of things that need to be removed through water changes.

Also, fish absorb many of their minerals/vitamins through the water column. Failure to replenish that stock of minerals means that they do not get replaced, and therefore the fish cannot absorb them.

Water changes need to be done to remove the nitrates and excess minerals/heavy metals and to remove other organic wastes.

No problem. Isn't the internet great

Hm. Thats odd that they dont carry them...since thats really the one test that should be done the most often. After awhile when everything is stabilized and you're on a set schedule, you don't really need to test much, but a nitrate test is a great thing to have.

I suggest you order one online, from someplace like bigals or another vendor. Simply because if there is some overfeeding/overstocking going on, or if you're not doing frequent/large enough water changes, the nitrates will build up and can reach harmful, if not deadly, levels. Anytime there's some sort of problem in the tank, the first place to look is often nitrates.

Or, you could periodically take your water to your lfs to get tested - assuming that they do it for free.

Weekly water changes are nice.. Every other week is the maximum time in between changes (IMO). I do know some go way beyond that..

There are many things in the water that accumulate that you cant test for. Even if the parameters you test for look good (ammonia, nitrite, nitrate) you should still do frequent water changes.