If you have any worries, get the Seachem test. The big difference between it and most of the others, including good old API, is that most of them raise the pH of the sample to about 12 or so, and then you get a measurement that is a total of both ammonia and ammonium. So it can look like you have an persistent ammonia reading, when in fact what you have is ammonium, which is not harmful. Only ammonia, free ammonia, is harmful.
I don't think it's necessary to age it, Prime is quite effective and will bind ammonia, as well as remove chlorine and chloramines. City water plants often add ammonia as well as chlorine, and they combine to produce chloramines. They last longer, so they work longer, and depending on the source water, may be used all the time or only at certain times of year. But it's rare that anyone will be notified of any change to local water treatments protocols. You can always contact the utility and find out what they use, just to know, but if you get readings that high from tap water, they are using ammonia in your water treatment plant, at least at this time.
Just make sure you treat the water with appropriate products before using it and you'll be fine. If you have space and time and want to age it first, of course, that works too. But where chlorine will gas out in a matter of 24 hours or so, chloramines take up to a week, or more, depending how much there is, to gas out of water. The larger the surface area of the vessel it's aged in, the faster gassing out will happen, and as Autumn Sky said, using an airstone can help by increasing gas exchange.