I prefer AU/Unc for the modern mass produced coinage that I take out from circulation. Of course, if I cannot find one, then a lower grade will have to do, but seldom will I go buy a modern circulating piece in Unc. I am much looser with my expectations with pre-WWII coins in this category.
For older coins that I purchase, it depends on the reason for why I am purchasing it:
-If it is part of a date set then I can go as low as F (though I try for VF-XF)
-If it is because I just like the design, then I aim for minimum XF.
-If it is a commem, then I usually hold out for AU/UNC.