
I agree, I would find out what the other dogs are like first - then at least you can avoid the dodgy ones so your bull only socialises with good dogs. That should avoid her becoming fearful of dogs in general - not something you want with a bullmastiff I'm sure!!
If you can't ask the owners for whatever reason, and anytime you see a dog anyway, I would put her on lead - then you can avoid encounters like you've had. It's frustrating having to do it, but I have to with both of mine - they are very friendly, but at least near me, the vast majority of dogs out for a walk don't appreciate having big, boisterous dogs in their face. Makes me feel bad too!
I would also start working on building a rock solid recall - then if she does play with a dog and starts pestering it, you can call her away and put ehr on a lead. If you can get it really, really solid, it could help you avoid potentially dangerous situations.