Using a master document to edit subdocuments will result in corruption.
Period.

Using IncludeText and RD fields bypasses the master/subdocument structure
altogether.
http://word.mvps.org/faqs/tblsfldsfms/includetextfieldscontent.htm

I would suggest writing back, in a separate topic (subject "RD fields and
Master Document workaround") with detailed information as to what it is you
are trying to accomplish. I would also suggest posting in the docmanagement
and formatting.longdocs newsgroups rather than in one dealing with visual
basic for applications. When you do this, post one message in both
newsgroups simultaneously (rather than duplicate messages in each
newsgroup). If you don't know how to do this, post in only one of the
newsgroups. http://www.mvps.org/word/FindHelp/Posting.htm.