I have one mean rottie mix curled up on my feet right now keeping me warm and keeping my blood from pooling in my feet! GRRRR!!
I wouldn't go out looking for a SD and choose a BSL breed, but if I had a BSL breed (uhm....like, I don't know...Harley) and they showed a natural ability to do the job and natural desire, I wouldn't rule them out based on their breed. It just means having to be must more aware of problems that you may encounter.
I got lucky in that my SD who I KNOW is 1/2 Rottie (was there when he was born and mom is pure Rottie) does not look like one.....meaning less stupid people comments.