Just set the motion zones to be your grass and exclude the sidewalk. That way when the dog that leaves the sidewalk to squat you'll get an alert.
Me personally? I want to see every person that comes down my sidewalk. You never know when that can be handy. People do the strangest things.
And as was said before, scrubbing an entire day to see whodunnit is easy, just fast scrub until the turd disappears, then hit play. Takes less than 10 seconds.
My cameras pretty much cover the whole front of the house. So if there is any dog poop it is easy to chase down the culprit. I look for when the poop is there and when it is not there and you close in on the time line very quickly. By the way, with the cameras I have only had one dog poop in about two years.