Well, if you are going to use Parallels, you do not need to partition anything. Parallels will create a virtual drive and it will "stretch" as you use it. (best deal here).
If you want to boot via Boot Camp, then you wil need to partition your HD. If you are not going to have many files on your Windows partition then, get the minimum space allowed by Boot Camp.
Hope this helps, but I would, honestly recommend using parallels. Cos, whenever you need to...