I not only heard that rumor.......and I have witnessed it myself, but I will admit that it was the Owner's fault not the Breed in particular. During my search for my 2 Yorkies (1 male @ almost 3 yrs. old & 1 female @ 2 yrs. old now). I would go up to strangers with Yorkies and try to ask them questions, since all the books & magazines give you the text book version of them.
The odds were for every 5 Yorkies, maybe only 1 was "Socialized" and the other 4 were outright vicious. But I personally could see past that, most of them re-act out of fear, insecurity and lack of Training! I firmly beleive that they are a reflection of me, so I made sure that mine were well "Socialized"
But it's definetely something you have to work on constantly every day. I just ignored what everyone else said when they tried to discourage me against them, but those same Family members all got Yorkies after they met mine! (Cousins & Sister) Not only are they biters, barkers/growlers with attitude problems and a bad reputation (said by non-Yorkie owners of course)
they are usually people that had a bad experience with an untrained & unsocialized Yorkie? I find that other people thinking about getting one always ask the same questions about their personality traits? I always encourage them to get one b/c I beleive they are the Best! Regardless of their Reputation, but I always stress "Training" is everything!
In fact I love mine sooooo much I have 4 now.......I went Yorkie crazy!