Often dogs that "only" agress other dogs while out walking have a problem with the confrontational head to head greeting that happens when we come upon another dog walking down the street. Although the food may distract her, it can also inadvertently reward the startings of aggressing. I like to make sure that my dog and the dog approaching him are not approaching head on, rather in a curved sort of fashion (think of an upside down C). This eliminates some of the confrontation. I also make sure that my dog is not giving any lasting eye contact to the dog on the other side. I do this by asking for a watch, or using a leave it and let's go to get him back where he's supposed to be.
There is a point where your dog will not aggress when passing another dog. It may be 5 feet away, it may be 50 feet away. You need to find where she is comfortable passing and is non reactive, but still aware, from there you can teach these alternative behaviors. Putting her above threshold all of the time is always setting her up to fail, and you are always doing damage control when you pass another dog. She can't learn when she's so worked up.
There are so many different collars/halters that you can use in your training....knowing your dog and more of the specifics of why she is aggressing, her triggers etc will decide which one is best for you and your situation. That being said, aggression is something that is best helped with an experienced trainer.
