In case it helps anyone, here's what I have discovered about the interoperability of Rating in WMA files between MediaMonkey (build 1708) and Mp3tag (V2.62).
MM UI offers values between 0 and 5 stars in steps of one half, plus 'unknown'. The display unit is stars: http://i.imgur.com/32OfdBv.png
MM file tag storage represents these values in WM/SharedUserRating accurately (w.r.t. by WMP 12 & Win 7 Explorer) in the case of 1,2,3,4,5 and approximately in the case of others: http://i.imgur.com/YfDigFR.png . The case of 0 (stored as WM/SharedUserRating=1) fails to accord with MM Help's implication that standard values are stored as standard, being 0 => WM/SharedUserRating absent. Forthcoming MM build 1712 is reported to "0 stars writes 0 to WM/SharedUserRating (instead of 1), so it will display zero stars in WMP etc."
They show in WMP 12 and Win 7 Explorer thus: http://i.imgur.com/b9XUav4.png
MM file tag storage represents these values in a proprietary field SBD/Rating thus: http://i.imgur.com/8mqa1Fd.png .
Mp3tag's RATING MM placeholder does not show MM ratings: http://i.imgur.com/6WOHU8O.png . This fails to accord with Mp3tag Help here and here. Mp3tag's RATING MM operates on a field "RATING MM" which is not used by MM, and so seems totally inoperative for MM Ratings. This is assumed to be a bug, reported here.
MM database export values may be imported into MM files tags as shown here.
Forthcoming MM build 1712 is reported to "0 stars writes 0 to WM/SharedUserRating (instead of 1), so it will display zero stars in WMP etc."