If you care about traffic, then you should post at least once a day. The more you post, the better you do. If you post often, you'll have more content on your site. The more content, the more search engine traffic you will bring in. The more traffic you bring in, the more links and readers you'll get. The more links you get, the more search engine traffic you'll get. And so on... Yes, I post at least once a day.
I post when I have something to say - I see no point in filling up blogs with meaningless content just because "I had to post something today". I try and post three times a week to Data Circle though.
I try to post daily in my blog when I can, but it isn't always possible. I think it is important to update regularly to keep visitors coming back, plus to give the spiders something to look at