I use XBMC (same scraper as Plex I would imagine?) and don't rename anything because it used to be that I was still seeding from that directory and didn't want to change anything, and it seems to get about 90% of my stuff without any trouble. TV shows definitely need to be placed in their own directories, and it certainly helps with movies as well, but it isn't mandatory. The only major problems I've had with movies was when I'd download a pack like Hellraiser 1-8, where they were literally named Hellraiser 1, Hellraiser 2...Hellraiser 8. I had to manually add those, but even then when I was doing the search I just had to put Hellraiser 8 and it automatically gave me the correct one as the first choice. It can miss some, most definitely, but not often enough to make me look for anything else. Oh, and it has a hell of a time sometimes with full bluray rips (all the stream folders), but I only did a couple of those just to try them. Fuck 20-50GB for one movie.
Stuff like Castle (2009) was definitely a problem, but once you name one show in the folder it assumes the others are the same.
However, now that I think about it, I might be using a setting where it checks MovieDB (or whatever it is called) instead of IMDB, because that seemed to work better. Perhaps you can choose which service yours uses too?
