I would recommend waiting till you have content and a running site. I've found that if you get indexed too early that it can be months until they get you reindexed as an open site and not as a soon to open site.
This can cause you to lose possible visits since people will see the indexed description or firt few line of your site as "under construction" or "coming soon"
I'm sure you know how important it is to get listed in Yahoo! - and let's face it; they are *very* picky! I'd hold off on submitting to Yahoo...I'd wait til after you've submitted to the other engines and you're pretty sure your site will do something to impress the reviewer.