Do not wait!
There are two reasons for this. Firstly, getting on the search engines has always taken a long time for a new site. Even assuming you do everything right, it takes months before your site is even indexed, and more months before it starts to rank well. As a rule of thumb, never expect to rank highly within 6 months of submitting your site to Google.
The second reason is a recent phenomenon called "Google Sandbox". Many SEO experts believe that Google 'sandboxes' new websites. Whenever it detects a new website, it withholds its rightful ranking for a period while it determines whether your site is a genuine, credible, long term site. It does this to discourage the creation of SPAM websites (sites which serve no useful purpose other than to boost the ranking of some other site).
By submitting your domain name to Google as soon as you register it, you are establishing a site history even if the site has no content. By the time you have built your site, written your copy, and developed the rest of your content (and written your business and marketing plans), Google will probably see no need to sandbox you.If you wait until launch day to submit your site, you will spend a month or two (maybe more) sitting in the sandbox watching potential customers spend their money elsewhere.