Is there a preferred process for getting a key from a client to access their home for dog walks and drop ins?

Each client has their own preference. Ask them during the meet-and-greet.

I've had two people choose to give me keys. They both live in apartment/condo buildings that need a fob and a house key so it is just easier for them to not have me call them at work to get into the building. They asked me to come over like a meet and greet and gave me the keys. I keep it on my own key ring to make sure I don't misplace it. Most people use lock boxes because it is safer and easier to use other walkers when needed. I wouldn't ever ask someone for keys to keep.