Posted: 22/03/2016 at 08:33
Clicking a lap button manually has no effect whatsoever on GPS distance accuracy or the ability of your Garmin to measure a km. However, if the Garmin has lost signal or something in that lap, pressing the button will for a couple of seconds give you an accurate time between the last two markers. If you have a scheduled pace then that could help you. If you have a watch that will display last lap in a data-field then you can check that at your leisure.
On balance I'd prefer to leave auto-lap on so that I have something to look at every lap, but at the same time I make a mental note as to whether I'm getting out of sync with the markers. But if you don't have a schedule why bother. A recent article in RW based on research argued that you shouldn't let things like this take away from the attention you pay to things like staying relaxed, thinking of form and technique.