If water is good, do you still change it?

You don't have to change water if ammonia and nitrite are at 0. If nitrates go above 20ppm you would need to do a partial w/c.

I would just keep an eye on it every day or so, and change 20-30% weekly if things remain the same.

yes you still have to change it, for the fish use up minerals in the water, and they are replenished via PWC

edit: i re-read the post... if the ammoina and nitrite are in check than you do not have to change the water more than the weeklys

I will start doing water changes every 7 days if water stays good - instead of every day (which I have been doing while cycling).

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Once a week is what I do now but when my tank was finished cycling at first I took readings every couple days to be sure all was right where it should be.
Keep an eye on the Nitrate now especially because now that yours is cycled, Nitrate will be increasing.
The amount to change weekly will be governed largely by the level of Nitrates, not soley by Nitrate levels though since there are other contaminants your removing at the same time as well as vacuuming.

are nitrates really bad for the fish? my lfs told me a long time ago that fish don't mind nitrates at any level, but i've heard that too much (more that about 20ppm) are bad

yeah, high nitrates for a long period of time is harmful to fish. if nothing else, they result in old tank syndrome where your fish become accustomed to poor water quality so that when you do a water change, the good water stresses them out and might kill them. most people aim to keep nitrates below 20 ppm as you said. if you have live plants, you add nitrate to feed them, but i think you still don't want it above 20 ppm for the sake of your fish.

If I test everyday and the water is good, I don't need to change it, correct?

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no, wrong. there are numerous contaminants in the water that you cannot test for. for example, there are a variety of dissolved ORGANIC POLLUTANTS, substances such as proteins, amino acids, phenolic compounds, pheromones (hormones that affect the behavior of other fish) and other metabolic byproducts which are continuously being discharged into the aquarium water by the fish. If these DOCs are allowed to become concentrated in the water, the health of the fish will suffer.

for example, laboratory studies have shown that high levels of DOC are associated with reduced fish feeding rates, slower growth, decreased reproduction rates, lower immune system activity and blooms of pathogenic bacteria in the water. it is now believed that many of the fish health problems thought to be the result of high nitrates are actually caused by DOCs.

in addition to those organic compounds noted above, inorganic byproducts of the nitrogen cycle in any aquatic environment also buildup in an your water. ammonia and nitrite are taken care of by your biofilter and converted to nitrate. this latter product can only be removed by dilution through water changes.

so while you may be thinking your fish are just fine, even without water changes, or if your water 'tests fine', in fact they are not. the most obvious example, in the absence of outright disease and/or death is how long your fish live. many hobbyists simply replace fish after they die without a thought to how long that fish should have lived. goldfish can live for 30 years, however i'd be VERY surprised if anyone on this forum or elsewhere has been able to maintain a GF for more than 5-7 years. mollies for 5 years? a red tail shark or other cyprinids for 10 years? Characoids (tetras) such as Neons, Cardinals, Bloodfins and Rummy Nose Tetras, Glassfish and Hatchet fish for 10 years? corydoras for 8 years? i'd suggest to you that you need to keep up your tank maintainance including water changes.

liv2padl & jm1212 read guppygal's post again. She has already stated that she plans to change the water on a weekly basis. She's asking if she needs to change the water BEFORE the week is up if no ammonia and nitrites appear before then.

Guppygal, weekly water changes are the recommended method. You're on the right track.

I missed that too, just for anyone else that does readin this thread, weekly water changes should be always be done for all of the reasons liv2padl mentioned.

I don't even check my parameters any more (unless problems arise), I just do my WCs and trust that to keep the water quality good.