Yes, you absolutely have to consider who will have to work with your files and how they will do it. If you just have to print them and no other computer has to see them, then it's not an issue at all. If people need to read them on their computers, you can save as pdf from any OS X app. The problem comes when people will need to open your files and edit them. Chances are you'll be dealing with Windows and Office in that case. .txt and .rtf will work in any word processing app, but if it's a file with lots of bells and whistles (tables, images, special formatting) you'll have problems with compatibility. I've found that going between AppleWorks and Word in these cases things get screwed up--even when saving as .doc. That's a case where you're better off using Word, but it's a rare instance.
TextEdit, by the way, will save as .txt, .doc, .rtf, and .pdf. To save as pdf go to the print menu. It'll be there as an option.
As an aside, does anybody still use footnotes? It was mentioned in another post. I've been writing for college classes since 1990 and have only been asked for APA style, which doesn't use footnotes. Well, it does actually but only in rare occasions and not for references--they're all embedded. Just curious.