Tried this with a previous startup. Soft-launched an iterated on the idea. A few months later, started pitching the idea to press. The reaction we got was very negative—no-one wanted to cover us unless there was "news". We had already launched so there wasn't anything "new" to report. We had to invent features to put in the product and sell that "story" (it worked, sort of: people wrote about us but reported on the new feature rather than the fact that we existed).

I recommend putting some sort of beta-wall or limited access on your launch if you're going to do this, if only for the excuse of being able to announce public access later. I wish PR tricks like this weren't necessary, but they really are.