This response is about how I would want to be treated and is based on how I treated people during my short lived online exploration.
It depends on the expectations of who you are talking to. If you are searching online, people generally include what they want in thier profiles. If they are specifically looking for a deep relationship and are looking to invest in you than I would tell them right away. That way there is no big surprise two weeks down the road after a connection has formed online or in person. I would consider holding this type of information back as a passive form of deciet designed to "trap" me because I am now emotionally involved and if I chose to walk away it would be more difficult.
I was very clear in my POF profile..."I barely want to know your name..I just want sex". Black and white. Those that contacted me understood 100% what I was looking for and I only got serious responses.
If you are completely upfront, you might not get as much activity but those that are curious and interested will be genuine at least in theory.
Now if soemone is just looking to hook up for casual sex then the importance of telling them becomes more a function of your expectations then theirs.
This is just how I would want to be treated again...not necassarily what you should do.