More than 86% of fatal dog attacks are done by this kind of dog:

[notadmblnd]

52. A tail between a dogs legs does not indicate fear

It indicates submissiveness. Nor does a tail up and wagging indicate friendliness. It indicates excitement. Also a dog backing up might indicate a lack of trust but not necessarily on the part pf the dog. If a dog's human is displaying a lack of trust, the dog will display the same behavior, especially if the dog is excited. When introducing dogs to each other, their humans should be calm but assertive. The dogs should also be in a calm state of mind. You can tell when a dog is calm- his ears are back, his tail is down or he is laying or sitting.

The way dogs introduce themselves to each other is by smelling so you want to make sure noses are engaged when greeting each other. A bark or a yelp or even a snarl while sniffing each others butts also does not indicate an attack/fight is eminent. It may just indicate that one of the dogs isn't ready to be greeted at such an intimate level. Yes, both dogs should be leashed during an initial greeting.