Although I use VS to edit my skins, I really don't see it as being more effective than any other editor.  You are pretty much just editing HTML and throwing in some tokens - nothing really VS specific as far as I'm concerned.  If I already have Dreamweaver open for some reason, I'll use it to edit my skins.

Don't worry about the errors on the broken references.  VS just can't find the file, but if you mimic the structure of the Default skins, you should be fine.

I only use the ASCX files.

Beside the question which IDE to use (for simple HTML/CSS/XML editing any text editor from wordpad to VS is acceptable, while some little helping functionalities like code highlighting, code completion/suggestion, commenting shortcuts, code outline, search functions, etc. can increase your productivity) I suggest to install the following two extensions for Firefox, although you should test your site on IE (and other browsers), too:

• Web Developer Toolbar
• Firebug