Do you check in and check out at every visit?

It depends. I know that Rover would like you to. They really want all of us using the "Rover Cards".

But me not being a fan of the Rover cards, I think that is something that i think should be brought up at the initial meet and greet. Some owners may want you too, others think that 4 times a day is over kill. If it is something that was not discussed at the meet and greet than I would at least the first day, and taper it down until you are able to talk to the owner and/or if she says something. Based on the owners response the first and second day, you should be able to get a feel on the owner. If she responds or makes a comment right away than I think she/he would be one of those clients that does want the constant communication. (especially if you send a pic and she reply back with "how are my fur baibes doing?") But if she does not respond back, I do not take it personally but I do cut back how many times I send them updates so I do not feel as if I am bugging them all day. If it was me personally I would send at least two pictures a day. as far as writing something with the picture will depend on how the dogs. If they were super excited to see me, were very skittish, or did something silly, ect than I would us that as my communication with the owner but if the dogs were the same, nothing exciting, nothing scary, just the same old same old than there really is nothing to write, a pIc will be just fine.
Good luck, hope this helped you decided. Cheers!!

It really depends. I ask the owners each time if I see they request them and majority of the time, they tell me they just want one card a day and I can leave the day's overview in the comments/notes section. 4 cards a day is quite a lot.