If you want to go the "low testing" route, you would probably be best served by getting some "indicator corals", softies, such as Kenya Tree and Xenia. These are very sensitive to water conditions, and very often serve as very visual "early warning signs" to let you know that something is off in the water quality.
The zoas closing could also be a sign of some kind of predator or other irritation. I would check them closely and see if you see any sundial snails or nudibranchs just to make sure you don't have an infestation and bring that to the main tank.
Also, you would be best served by doing some kind of testing once a month or so, just to be sure that your parameters are in check. If you salinity is wandering dangerously off course, and the zoas may just be acting as your "early warning system" (something's eventually gotta tell you that something is wrong).
In that case, if you let the conditions continue to slide, the rest of your system can get into jeopardy. Reef-keeping can take a lot of time and effort, but a little laziness in the wrong place can cause more problems later on.
Testing will only take about 10 minutes to run 3-4 tests, and with a refractometer you can get a very exact reading of your salinity in only 30 seconds -- time well spent.
I don't like to do a lot of testing, but you have to do some, if just to get a feel for how your systems are running.
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