Key lock boxes are a terrific way to handle access to a residence. They are readily available at hardware stores and amazon. Real Estate Agents use them also.
My clients have mounted them outside the house, used the go around the door knob kind, and given me the code to their garage door, and then you can have separate lock box inside - or access through door. I do NOT like having a key to person's house full time - and I now refuse to take one. I explain safety wise it is better for them to have control of the key. should they have other contractors coming into the house - we can all use the same key. When someone is no longer authorized to enter the residence, change the code.
An interesting side benefit, is the times when the resident locks themselves out of their residence - a key is right there - no more having to break into your own home. It also allows me as the sitter to respond from anywhere - should the owner get stuck in traffic, the meeting that won't end - stuff like that. I don't have to worry about going home, getting a key, which vehicle I left the key in and all that.
I now require it of my clients. Clients who don't wish to, well.....that is their decision. I'm not a fan of people "just leaving the door unlocked". it really just isn't the best - and will often mean the insurance company does not have to pay for items stolen/trashed in a burglary. The other benefit to you is when there is a break-in or some other incident where the police ask "who else has access to your house"? - you only have access to the house when you are at the residence - and were across town - walking another dog etc.
Remember, you set the policies for YOUR business.