IMO, the ideal meeting would be off-leash (maybe muzzled) and on neutral territory. To protect the dogs, it'd be ideal to have an exercise pen between them so that they can sniff each other without getting into physical contact. The reason I'm so anti-leash-meetings is because the leash restricts movement and prevents natural dog greetings. Instead of taking away the stress, it increases pressure and stress.

The ideal meeting often doesn't happen, though, so when the two dogs DO meet, make sure your leash is ALWAYS loose (you might even want to drop it on the ground and let the dog drag it - that way, you can quickly pick it up and pull your dog out if there is trouble). Other than that, see if their bodies are tense or calm and "floppy". The former means trouble. The latter is a positive sign.