Both sides of it. When I worked at a vet we did request it if needed, but it wasn't often. The downside of having client do it is that if they don't do it right the muzzle can be removed by dog., we had a huge aggressive GSD this happened with and it was really dangerous. As a dog owner I tell them to muzzle my senior dog, but the rest are fine. If I thought they would bite I would tell them to muzzle.
Misty- Samoyed Mix, Tannor- Golden Retriever CGC
Robyn- GSD CGC, TC, Midnite-GSD CGC,TC, Brennan-Golden Retriever CGC, Batman-Husky/Greyhound , Apollo-GSD