A well set up and established tank,with light stocking levels, good filtration including UGF, and lots of plants, can go for a lot longer than a week without a water change and still keep zero levels on the ammonia and nitrite levels. That being said it is still best to do partial water changes every one or two weeks, if nothing else you can use the time to really inspect your fish and look for problems in the tank.
Yonk,
I think you should go ahead and check your water parameters, ammonia, nitrite, nitrate, and ph, and then compare them to your dechlorinated tap water readings. I am betting that at least your nitrate levels will be high. Elevated readings, even if not in the levels that cause immediate damage, will lower your fishes ability to fight off disease, prevent the survival of any spawn, and in the case of long term exposure to high nitrate levels, open the door to HITH infections (esp. in larger cichlids).
The kits are not expensive or hard to use and improving those parameters will result in healthier, more robust fish that will reach a larger adult size.
The ideal readings are ammonia 0.0, nitrite 0.0, nitrate 20.0 ppm or less, pH varies for the type of fish, what you are mainly checking for is any change between your tap water and your tank.
