Ping sites more than once and you risk being banned – the ‘big’ lists usually hit the multi-pinging sites (like pingomatic) so being banned is very easy.
Also, even if the list be huge, some might not be appropriate for your blog – say like a site that tracks only Japanese blogs (Assuming your blog is not in Japanese.)
Pingomatic covers the important once, and like Podz said you only ping once 🙂