As frankz has already pointed out, all media players are writing to the same tag (POPM in mp3, RATING for VorbisComment, WM/SharedUserRating in wma)
so unless Steven implemented the POPM tag as per the ID3.org standard -
The purpose of this frame is to specify how good an audio file is. Many interesting applications could be found to this frame such as a playlist that features better audiofiles more often than others or it could be used to profile a person's taste and find other 'good' files by comparing people's profiles. The frame is very simple. It contains the email address to the user, one rating byte and a four byte play counter, intended to be increased with one for every time the file is played. The email is a terminated string. The rating is 1-255 where 1 is worst and 255 is best. 0 is unknown. If no personal counter is wanted it may be omitted. When the counter reaches all one's, one byte is inserted in front of the counter thus making the counter eight bits bigger in the same away as the play counter ("PCNT"). There may be more than one "POPM" frame in each tag, but only one with the same email address.
<Header for 'Popularimeter', ID: "POPM">
Email to user <text string> $00
Counter $xx xx xx xx (xx ...)
with email addresses to hold separate player ratings, then it just wouldn't be feasible.
If you have 'store ratings in the music file' unchecked, then your original Winamp ratings are not going to be overwritten. Mb will save any changed rating to the database and display it, but the original rating stored in the file can still be viewed in the Tag Inspector.