Each owner has the right to say a dog has to stay on the lead, but only you have the right to say a dog can come off the lead. It depends what you are comfortable with. I let most dogs off the lead immediately if we have permission, but we only walk 4 at a time, and the other 3 will be familiar dogs. Some it's taken the owners a long time to give us off lead permission, and I've worked hard to get it because I know the dog. Other dogs we've got off lead permission for, and we keep them on!
When you start up, you'll build up slowly, so you'll have time to get to know each dog and learn as you go. I started dog walking from two years fostering so I already had more experience than many.
We don't take very nervous dogs off the lead, especially small breeds, no matter what the owner says. These are the dogs most at risk of panicking and running away, and being small they are very hard to find, and most at risk. Fear is the reason most dogs will run away - being too friendly with other dogs is the main (but not only) reason they run off - two very different things.