Best practice of split the disk space for MacOS and PD VM(WIn8)?

Hi, considering all kinds of situation: file safety, disk space utilization, time machine or CCC backup, expandable.....
How to plan the disk space for Parallels Desktop Windows8 VM? Assume half of time needs to work under Windows 8.

From Windows8 in VM, it can access the MacOS folder
From MacOS, it can access the folder in Win8 VM folder as well.

Should I put most of files to the MacOS disk, or put them into Win8 VM?

Thanks a lot

 

* Keep the files on the Mac side and share mac folders with Windows (it shares by default).
* You don't say how big your disk is, but doesn't matter, just accept Parallels defaults.