shift is probably only necessary where there are two menu options that are the same except for shift. in other cases, the shift is probably unnecessary but acceptable.
this may be true of other modifier keys as well.
I tested it in activity monitor because it's relatively sparse on command keys, and found that cmd-1 to open the activity window would merely bonk with any other modifiers held (shift, ctrl, or option) so guess not. They must have coded mail to allow shift also even though there's no clear indicator. They may have accomplished this via a hidden menu item, because programmatically (within their public frameworks) I don't think it's possible.
