Always before shooting to make sure they're high enough to have insulin at all
I'm not that familiar with Caninsulin since most here don't use it, but it generally doesn't last a full 12 hours, so it's nadir is probably in the +4 to +5 area (so 4-5 hours after the shot) so you'd want to get a test in about then to try to catch how low it's taking Murphy
You want to get at least 2 tests in on each cycle every day (so 4 tests/day) The Pre-shot tests, and on the AM cycle a test around nadir and at least a test at night "before bed" to make sure it's safe to go to sleep (most cats go lower at night)
As for curves, you don't really have to do a perfect "curve" but of course you can....maybe once a week get a test every 2 hours for 12 hours or every 3 hours for 18 hours....otherwise, if you just get spot checks at different times, you're going to get the information you need without scheduling a specific "curve"
Think of the spreadsheet like it's a puzzle. If the only pieces you have filled in are along the edges, it makes it impossible to tell what the picture is....the more "pieces" you have sprinkled throughout the puzzle, the easier it is to see a clear picture of "the puzzle"