I agree that it's a big problem. While I don't think you should be messing with your dog on purpose while they are enjoying some alone time or private time, etc, you SHOULD be able to if needed. I cannot even fathom if Jackson growled at me, or showed his teeth, or anything. And no, I'm not his "alpha" -- but we have a mutual respect for each other.
Now, growling at other DOGS is a bit different, IMO. Because they have their own little language and it won't bother me if another dog is getting in his face while he's chewing a bone and he gives a warning growl to go away. They are animals after all. I agree with Ringo1 in that prevent the problem by picking up all bones off the floor so they don't have the chance to squabble. But when it comes to HUMANS, no, I do not think tolerating guarding is okay. We are feeding them the food, we need to let them know that we're not out to take it away from him, we are the GIVERS of food, lol. They need to trust you.
Really, I think a behaviorist could be in order. Resource guarding and food aggression is not something to be taken lightly and it can turn very dangerous. I hate that so many people think it's adorable when Yorkies act like this (I've seen sooo many YouTube videos of little dogs growling and freaking out if you touch them, or try to take a bone away, or a toy, and people are just cracking up. I do NOT think it's funny at all, because if a German Shepherd were doing this, people would be all up in arms and talking about putting to sleep). Not saying you are this way but it's a generalization I see and it's often looked upon as something cute or not a big deal with little dogs whereas I think it's a huge problem.
You could try to implement NILIF.
http://k9deb.com/nilif.htm