Even though having "Green Day" and "green day" automatically shown under just "Green Day" in the library is convenient, it doesn't alert you to the fact that some tags are lowecase. The problem of having "Green Day" and "green day" shows up most when you sync to a device that doesn't automatically group the two together, so you see two entries for the same artist, except that one is uppercase and the other lowercase.
So maybe an option for case-sensitive grouping?