I have always carried my mug and refilled it at whatever resort I was at. It's allowed and you pay for a resort refillable cup. They say it's good at any resort.
In the parks if I bought a soda I used the mug to hold the leftover (because I don't like sweaty paper cups). However mostly I used it (in the parks) for water since I could fill it with ice and water pretty easily at the walk ups.
I'm more of a water with meals kind of person so I never really thought about using it at a TS.
Would I take it into a TS? Maybe if it already had something in it, but I'm not sure I'd intentionally fill it with soda just to avoid buying a soda at a TS. Not because I think it's tacky, cause I don't. Just because I mostly drink water with meals and a Coke Zero is more like a cold caffine snack for me. LOLOLOL