Frosty is a lot less predjudiced than I am. So I use her as a bit of a guide. And I pay attention to the behaviour of each individual dog we meet.
I try not to be scared of the bigger dogs - the sort that could kill in one chomp if they wanted but it's hard because you don't get much time to find out if they're a good dog or not. And I try not to make assumptions about dog owners based on their dogs. You don't know if they chose the dog or not. I met a nice mum with an undesexed pitbull - about 8 months old - it was a well behaved dog with a slightly frazzled owner - turns out the dog was chosen by her ex - and will soon be desexed. It still scared me a bit but it was well behaved and Frosty liked it. So much for BSL.
I've met lots of big boofy blokes walking small white fluffies including one - who picked the dog himself and won't get it desexed because he thinks it's cruel - you can practically see him cross his own legs when he says that. He also understands that what happens to the unwanted puppies is also cruel... but so far - fail to do anything about it. If you saw these blokes in pubs - you'd swear they'd have big boofy dogs but no, not true.
The worst behaved dogs I've ever met belong to families with small children and well meaning air heads for parents and I worry for all of them. And those dogs have mostly been of sight hound extraction, with one bad rottweiler and a couple of bad labradors.