I totally trust my dogs off-leash but we do 90% of our training off-leash from day one. But like you I use discretion and common sense. Just because I trust them doesn't mean I'm not going to be wise and cover my bases. My vet's office has an odd parking situation, right off a very busy street. Your car is literally about 6 feet from the road when you are unloading. I know my dogs will hop out and platz on command, but I still snap a leash on while still in the car. I've never needed to really use it, but it's on regardless.
We have a private 12 acre dog park as part of our business so my dogs get a ton of practice coming to me off leash when called with TONS of distractions. It's a pain in the neck sometimes, but it is a nice skill when you've worked on it to be sure.