The answer is: "It depends." You can do well with a set of Mercury Dimes in Uncirculated 60 - until you get to the 1916-D. Generally, based on what I have been told as regards exhibit judging, it is considered best to have the coins be within the same grade range so that they LOOK like a set, rather than an assemblage, say Seated Quarters in VG to VF with Fine as the centerpoint; or Fine to XF with Very Fine being the midrange for most examples. It depends on how you want to display them, also. And nothing prevents have a showpiece such as a variety in higher grade within that collection. It is all up to you, of course.