No, you can skin perfectly fine without a sub. I did so for years whilst slumming in HTH.
You download the software, get it up to the latest patch (including latest skin viewer for your version). Get the hi-res tex file, and tada! All the tools you need to make a skin (aside from an actual graphics editing program, I mean).
You know you can play with your skin offline and test it out, take screenshots, etc...? You only need a subscription to hit the online arenas.
EDIT: 2 extra points.
1) You don't need custom skins to create your own custom skins. You just need the hi-res defaults to use as a template.
2) You can download the skins off the AH webpage and plop them into your skins directory (they are *.res files) and view them offline as well. The in-game download just does that for you, but you can pick and choose those you wish from the main AH webpage ("Downloads" section)
EDIT2: here's the link:
http://www.hitechcreations.com/download/skins/index.html