Excluding it from further back ups means it won't appear in TM later backups and would eventually be deleted when TM trims backups, that's how TM works. Really, TM is good for backing up OS X, it's optimized for that, it has no business backing up Windows, Parallels provides a solution for TM integration, but if you want an optimal solution you should use a Windows back up tool, you might or not copy your VMs initial state, but a robust Windows backup solution will be able to recreate all your Windows install in a new an empty VM anyway.
As for me, like I said, i just copy it from time to time, and keep the current copy and the one before. My files and documents are all on the OS X side anyway (shared folders).
Last edited by a moderator: Dec 4, 2013