Something you should know before you go tagging all your media from within iTunes is that the information you add gets added to your iTunes library file and not the media file itself. If for some reason you move/share your media or try and run it in something other than iTunes, all that laborious labelling will be for nothing.
This is not entirely true.
For WAV files, PDF files and maybe another format or two, yes.
But for mp4/m4v files, things like artist, artwork, comments, disc number, track number, season number, etc. stay in the file.
Ratings, video type (Music, TV Show, etc.) will not be copied unless you use a program like MetaX or something.
But the most important information IS embedded into the file. I use MetaX for tagging my movies with Actor/Producer/Rating, etc., but all of my TV Shows are tagged directly in iTunes. I have ripped about 15 - 20 seasons of different shows (Probably over 500 episodes in total) and every last one of them has been tagged in iTunes and removed and re-added to my library without me ever having to retag anything.