After working for a mobile grooming company and having my own Yorkie done by other groomers, and then finally doing her myself, Ive learned alot.
First of all, I made the mistake of telling the groomers assistant what I wanted and ended up with something I DIDNT WANT at all because the assistant neglected to tell the groomer. So talk to the groomer directly. I was SO angry, I cried in the shop! They cut off her top knot and cut off 3-4 inches more then I said, GRR! Ahh anyway...
You can also tell them what shape you want her head/beard, round or rectangle, if you want them to cut that area.
When I groom my Yorkie I just trim off what areas seems thin on the body and head (or "even" it up), round her feet, and trim up her backside. It gives her a pretty neat appearance when I keep on top of it all.
We also tried not to do much when it was a puppies first or second groom, so they hopefully get used to being handled in that matter.
I just found out that I was WAYY too picky with what I wanted, and Groomers always tend to do what they think looks best. So I got passed being nervous cutting her hair, realized that it WILL grow back, invested in some grooming tools, and now would never have someone groom my furbaby.