I don't think you can make any one-size-fits-all rule about this. In the end, it comes down to what the teams and creators feel comfortable with. Some like to do an all-out "ad campaign", if you will, others like to work away in secret. Both approaches have their pros and cons; While a secretive project will come as a (hopefully pleasant) surprise to many (Ransom's Windmills, for example), it is also less likely to attract random people wandering in and wanting to contribute. "Public" projects, on the other hand, have to spend at least a part of the development time on publicity matters, if only to make sure that people are still aware of their existence.
Expectations are a two-edged sword, really, while they may backfire on you if you don't make an effort to live up to them, I know a few people (myself included) who work better under such pressure. Having to deal with people asking "When will it be done? Can I haz? What about nao? I can haz nao?" is, in my eyes, a minor price to pay for a good motivation.