Some people prefer not to because it can make your soil stay too wet. Others think it robs too much of the fertilizer and nutrients from the trees. Some like the clean look of nothing growing in the mix except the tree. I personally prefer a light top dressing of mosses and other micro plants because it looks good and helps keep the soil from washing out in our pounding and very frequent rains. But I also have to keep an eye out so the media doesn't stay too wet. If you are in a drier area, it may be useful to keep your mix from getting dried out as fast. If you are in too dry an area you may not be able to keep moss alive.
I would suggest if you use it, keep it off of the base of your tree and don't let it get thick. Pluck it out regularly if it does and only leave little bits to regrow. If possible choose species of moss that grow thin and short rather than in thick lumps. You'll want to be able to see your nebari, the base of the trunk, and any larger surface roots anyway. I'm personally not concerned about the moss taking up ferts and nutrients because if you are really concerned about it you can just add a little more.
As you can see, there is not an easy answer to this and you will get some people who swear by having it as your top dressing, and many others who swear not to allow it at all except for show display. Really you are going to have to decide what you prefer and more importantly, what your trees and environment dictate. And it may vary from tree to tree! One may do better with the moss because it likes to stay wetter and another may really struggle because it needs all the air and drainage it can get. For me, where I live, there is NO STOPPING the moss. Moss and lichens are everywhere up here and will grow on anything that doesn't keep moving fast enough. So I go ahead and add it and keep it thinned out regularly. I basically live in a rain forest up on a mountain. Your growing culture will vary greatly from mine.
