It really depends on your product and objectives! Are you pushing for users, sales, or simply awareness? Are you B2C or B2B, and what kind of approach do the types of people whose attention you want tend to see from competitors/substitutes? Think these goals through and figure out where the marketing takes you. I personally tend to love doing a heavy PR push from launch if the idea/founder(s) is(/are) really media-sellable - how's your story? That said, I would never do this kind of a push without at least a few weeks of beta to work out kinks/whatever. Either way, if you craft the story properly, you can get the kind of attention you want - good luck!