Can I ask my doctor to inject saline in my face to simulate Restylane/Juvederm or is this useless?

You are a smart cookie. This is exactly what I do for my patients when I am not sure whether to use a filler in a certain area or not. Saline is safe and goes away in a day. RegardsDr. J

Yes you can do this, but I don't see a reason to.  The HA fillers you talked about, Restylane/Juvederm/Belotero are reversible and if you don't like them they can be dissolved in the office without any consequence very quickly.  More injections means more swelling and bruising, I would find a dermatologist who performs filler with cannula for the tear trough.  Then, you have almost no risk of bruising or swelling and if you don't like it, it can be reversed!  Simple as that.  Best of luck, Dr. Emer.