Even better: If possible, don't ask for a signup until the user has actually tried your product. Just create a guest account which is deleted if unclaimed for a week or so, and disable "sensitive" features that could be abused.

I have a couple of sites where I allow them to use the product just like a signed-in user, but to save their work, they need to sign in. So people can play around and try things about before entrusting me with their email.