Say you have Win8 RP in a VHD. You want to create a replacement VM for RTM. Once you delete the VHD with RP (or more likely move it).
If you do this, allocate all of the space in the new VHD. OS X seems to struggle with expanding out that file into reclaimed space. Also, once expanded, OS X seems to struggle with using the space that may not be contiguous on the disk. (Even with an SSD.) If you expand all the space out, you should be better off. Word of warning though, may take a couple of hours to create a ~100GB disk.