My Rott female is doing well in obedience, but has started to growl at 1 in 10 people that approach. Do you have any advice?

Hello,

I have a question with regards my 17 month old female Rottweiler. Jasmine is spayed, 72 pounds. She came from a reputable breeder of rotties in my area who breeds working rotts for schutzhund. He gave Jas to me rather than selling her as she was the runt of the litter with the stipulation that I have her spayed. I am currently working with Jas with an eye towards competing her in obedience trials. She and I are in a basic obedience class now and she is really doing well. Loves to work with me and very quick to pick up anything new. She has 2 issues, one when on leash she tends to be defensive with other dogs, barks and jumps towards them. I have been working with her on this using a prong collar correction and "leave it" She is much better and now works well in class paying attention to me with only an occasional glance at another dog. The other is when approached by a stranger she is fine, sits quietly at my side no problem its when someone she does not know tries to pet her she will lift her lip a bit. She doesn't do this with everyone, just maybe one out of 10??? She also does not want them to touch her feet, sometimes a low grumble. My question is this. Do I use a firm hard prong correction with this behavior?? Or should I work more in the direction of her seeing strangers as a good thing, treats etc. I have had advise from trainers on both these methods and am a bit confused as to which tact to take. She is fine off leash in our home with people.