The topic of optional and default prompts is probably one of the most popular topics here on BOB. I suggest a search for those terms, and suggest that you take a look at the Designer FAQ as well. The answers will also differ depending on whether you are using Reporter or Webi.
With Reporter, it is not possible to leave a prompt blank, and there is no generic method for defaulting a prompt. Again, your search will turn up many workarounds however.
Dwayne Hoffpauir 
(BOB member since 2002-09-19)