I suggest you put adsense in your site when it has some content. No need to wait for the time when you will have a certain amount of traffic, putting adsense will not harm your site.
Placing adsense ads on sites with poor contents (that happens during the construction stage of any site) will mar the reputation of your site. So wait for the site to be loaded with good contents and traffic and then you can place your ads.
Try to put none or very few (one) on the first page of your site, and then normal one the other pages. So that when you start your site, you'll submit to web directories and some other places, the webmasters will only look at the first page and the general look and then decide to accept or reject your site. So by having very few ads on the front page you increase your chances of being accepted in web directories.
i think having a little block does not harm in filling up the blank space for your website.....but not too much