It really depends. The survey is an interesting method of filtering people in of itself, to assure they'll use the service AND it has the added bonus of giving you some free market research. However, you don't want to be too intrusive at the risk of pushing people away.

Frankly, I think you'd be more than fine if you let it be on a first come, first served basis, with the added caveat of garnering a little market research. That way you're not turning away too many people.

You run the risk of taking on a lot of work if you start filtering everyone. And if you do that, you may miss out on a potential market you hadn't identified previously. Just a thought :)