Here is a great post on the VertigoSoftware blog on where the best place to store documents in TFS are. As they point out there are three locations where you can store documents, on the associated WSS portal, as a file attachment on a work item, and in the version control store.
The key piece of wisdom? It depends on who the intended audience is.
I must admit that I use a similar technique – although I find that using work items is the easiest (for me – but I am often my own audience). After that – if I need to share them outside the development team I will use the portal site.