Does it matter at all which
doctype I specify in an HTML email, or if I specify one at all? Will it affect rendering?
I used to use none, XHTML 1.0 Transitional provides slightly better results.
Some info and other useful info as well here: http://www.creativeglo.co.uk/email-design/html-email-design-and-coding-tips-part-2/
I use HTML 4.01 Transitional and have found that it gives the best results. It is very allowing standard in what attributes I can use, which is often needed when creating HTML-emails for various email-clients.
It's good to remember that alltough graphical HTML-emails still is total darkness in terms of web development, you should try to follow standards as best as you can without the layout breaking (and in the end making the content inaccessible).