Should I run Parallels off of my SSD or my internal HD?
I put it on my SSD, like all other apps.  I think the advice to put it on a separate drives is for when you only have hard drives - 5400rpm at that (slooow).  With Visual Studio being so slow (been using it for years), it can certainly use the speed boost.  Mind you, I just set up my iMac on Friday and set up Parallels this morning, but so far, so good.  Windows apps are running very fast, and so are Mac apps.