Our company handles this a bit differently. We don't ask the "How did you hear about us" question on the sign-up form. Instead, we wait a few days after the person has gotten into the demo, then we send an email from a salesperson saying something like "Hi, my name is Jack. I'm the customer support rep assigned to your account. Feel free to write or call me if you have any questions. BTW, how did you hear about us?"
It's amazing how many people don't want to talk to anyone they perceive as a salesperson. But they are more than happy to provide us with a simple bit of marketing data. And then since they are writing the email anyway they ask for a price quote, or tell us what they don't like, or whatever.
The bottom line is that this gets the relationship going. Once the relationship has begun, the sale is nearly won. YEE HAW!!!