I'd never trust any company who just pops in because they're doing work in the area. This is an approach taken by the less reliable companies, and as a result, the better ones tend to avoid the technique. The better ones also usually don't need to as they'll be kept busy through word of mouth referrals.
Even where dealing with a reputable company, you've no guarantee that the people calling to the door are related to them, or trying it on with someone else's good name.