Ask YC: Is it never too early to invite press coverage for a startup?

We're launching our private alpha next week, and I've been in touch with journalists and bloggers for a while now. Of course, everyone wants 'an early look', but is it ever too early for press? We believe in releasing early and releasing often, but am worried about showing a site that's too raw to a prominent blogger/journalist. First impressions count for a lot. What were your experiences like?