Many seo's will tell you that submitting your site to a search engine more than once is considered spamming. I've also been told by seo's that submitting to dmoz more than once is like being in a really long queue, leaving the queue, and re-entering it at the back!
If I submit at all (its not usually necessary, the way google and yahoo are going at the moment, as long as you have links to your site you will be crawled) I only submit once and that does the trick.
I suggest you check with some seo's though.