I figured out how to do this last night. I thought it was awesome and now I have something else to waste my time doing. If you are not OCD like me, feel free to just ignore it. (=
First, you go to the song you want to tag
In the "Comments" box of the info tab, type in whatever tag(s) you want the song to be filed under. You can do this for multiple songs at a time if you want to tag someone's entire discography the same way or something, just select multiple, then hit "yes" to the box that pops up.
So, now that there is something in the comment box, you are going to put it in a special playlist. Go up to "File." Right under "New Playlist" there should be an option for a "New Smart Playlist." Click on that one.
Now you have a lot of options. Click on the drop-down box that currently says "artist" change it to "comment." Keep the box that says "contains" as-is, then just type your tag name into the box.
Make sure the box for live updating is checked, this will update it automatically.
You can also change it so it will only include music that have two specific tags in them by clicking on the little plus sign, then setting it up for two tags. You can also make a playlist with songs from either of those 2 tags by changing the "match all following rules" at the top to "any following rules."
I just thought that was cool. I'm sure most people won't use it, but like I said, if you are music OCD like I am, it might be fun. And it's better than putting everything into manual playlists.