I wouldn't use it for ammonia. If you have any levels of ammonia, you may be doing something wrong. Perhaps you do not have enough live rocks cultured. You can use inaccurate test strips for other tests, but definately use a more accurate test on a regular basis. But, I think what you find, if the test strip is positive, then it is useless if you have to followup with a real test. So, why bother? Just do your real test when you have time. After all, positive or negative, if you are in a rush for a test strip, you probably are not going to take the time to fix your problems with a water change or something. So wait until you have more time to do it right.
In a tank with sufficient biological bacteria, the ammonia should be transformed into nitrate within seconds to a minute. Your tank should not have any traceable levels of ammonia at most times, unless something passes away and is constantly releasing ammonia where it is too much for the existing bioload to absorb. Some may report a spike right after feeding, but generally it is undetactable after a short period of time.
If for ammonia, I would recommend seachem's in-tank color changing device. It lasts about 6mo - 1year, and costs about $8 online. I think in-store charges $15. It changes color when ammonia is detected, and left intank. Very easy and convenient, and I believe it is pretty accurate, but I cannot confirm it.
For a decent saltwater tank, regular ammonia testing is useless because fish and critters cannot live long if there are ammonia levels. Some fish may seem stressed, but I do not believe they will survive too long. So a dead or sickly fish may be your first warning before you can take out that test strip to give you a positive warning. I think ammonia testing is only useful during the cycling process, after that, I am not sure why you would ever need to it unless you suspect something is wrong.
If there is something wrong, I would also test for nitrates. If you have an ammonia related problem, your nitrates will be very high, too. I do not think it si possible to have high ammonia and zero nitrate--that would suggest that there is zero beneficial bacteria in your system (which you may find a dead critter as the first warning before you are able to react to use a test strip).
-Kyle
