That's actually something suggested during startup school - make sure you get customers to pay for a demo (if at all possible).
We have people signing up with the credit cards, and we're still building out the MVP so... We're relatively content at the moment. Plus, we're really only looking for people who have incentive to say:
"Hey, your product sucks you need to fix X, Y and Z or I'm out!"
I'm not a big fan of blanket advice like that. The best sign up flow is the one you test for your specific product. "Collect credit card information ASAP" is a meme that works sometimes, but sometimes doesn't.
We have people signing up with the credit cards, and we're still building out the MVP so... We're relatively content at the moment. Plus, we're really only looking for people who have incentive to say:
"Hey, your product sucks you need to fix X, Y and Z or I'm out!"