I always board last but boarding last and wasting time at the airport are two different concerns. You can get to the airport way too early and still board last.
The way I'd say it is "if you've never missed a flight you're wasting too much time in airports". More of a long-term outlook. I generally see it as a failure if I have enough time at my departure point to visit a skyclub, for example.
This last point is kind of how I see it from my home airport. I have my routine timed well enough that ATL SkyClubs serve more as free coffee bars than anything else.
Aside from delays, if I have a need to find somewhere to sit in an ATL club, I've arrived at the airport way too early.