For traveling sitter service, I'd never mix dogs without owner approval. In my experience, the clients that need their dogs walked at their home, have dogs that although bonded and nice with my dog tend to be territorial. In my opinion, entering a home with a stranger's dog invites trouble.
For boarding service at my home, the answer is different because I'm introducing them at my home and they're spending time together and I'm observing their interactions. Still, for safety reasons, I'll often split up the dogs I'm caring for into many walks - and foregoing saving time. If I determine the dogs can walk nicely together And I can handle the pack knowing the many possible occurrences (other dogs and humans walking, landscapers working, etc.), I'll use different fixed length (not adjustable) leashes in different colors & textures and remove any waste bag dispensers that add to leashes getting tangled.