A personal preference.  Personally, I like seeing trees in the middle of my lawn have a nice bed of fresh mulch.  But are there benefits, you ask?  Of course there are.  First, by cutting a circle around trees in the middle of your lawn you hinder the lawn maintenance guy(s) from taking the ole’ string trimmer out and whacking the heck out of the base of the tree.  You are correct, so you hit the tree once with the string trimmer…not a big deal.  But years and years of consistent string trimmer strikes will start to remove a ring of bark, hindering the trees ability to take up water.  Second, it just looks nice!  Look at the example above.  Imagine if there was not mulch.  Imagine grass growing 20” up the trunk…yuk, what a mess.  The mulch bed is a nice, clean look.  Third, well moisture.  If you take the mulch bed out as far as the drip line of the tree, well you’ll keep the root zone of the tree cooler, and the mulch will help keep moisture in.  This will definitely benefit the tree.  And most importantly, don’t pile the mulch up too high around the trunk of the tree.  In fact, the mulch around the trunk shouldn’t exceed 1-2” in depth.  If you do pile it on, it will start to decay the base of the tree.  For more information on mulching around  your trees, feel free to contact Joe Yedowitz, owner of Emil Yedowitz Landscaping and Irrigation Solutions, at 914-377-9039.