I think there's a lot more in play then just if the user has only responded once. I think the whole account will be looked at. Do the reviews come in at random? Or, do 5 come in at once? We've got a few clients with local accounts and are doing great with just a reviews from people who haven't reviewed anything before on Google.
For Google, we've never had a problem with reviews not sticking. I've never used Yelp before for the reviews though.