The honest truth is, if I see another dog when we're out I get him straight back on the lead. I didn't like any dogs, adult or pup coming and launching themselves at my last dog... most of the time it's fine, but he was ill for a bit which made him very grumpy when other dogs came bounding up with their owners yelling 'oh don't worry my dogs fine' and I'd have to say 'well mine isn't he's in pain at the moment and likely to snap which is why he's on a lead' it was very tedious. I want to encourage Dave not to want to play with other dogs when he's on a lead or they are. I am very happy for him to play with other dogs in a controlled environment when we know the dog or have talked to the owner first and got their permission, but not really random dogs that we meet when out walking.. I had a nightmare with my last dog who never really grew out of the enthusiastic puppy stage and would bolt off the minute he saw another dog if I didn't spot them first - it was my fault that happened and I'm determined not to have a dog that does that again! It's not fair on my dog or other walkers IMO...
Your best bet, IMO, is to arrange to meet up with some other people who want to socialise their dogs and do it in a planned way.. Dave has had some great play dates with other dogs that we've met through this site and that we know locally which he gets loads from in way that's safe for all the dogs

Hannah x
ETA: With ref to my last dog, the bolting off as soon as he saw another dog was largely to do with his very over enthusiastic character and him being a but hyper and highly strung.. However, he got turned on a few times by dogs that didn't appreciate it (quite understandably) and that in turn caused a very happy sweet dog to become a bit aggressive towards some dogs later in life. So I'm not saying that letting your pup go up to strange dogs will defo result in a problem, but the potential is there if they have a bad experience from it and I don't really think it's very fair on the other dog/owner.