I will side with KC that check lists are necesary...
Many examiners will fail you if you fail to use check lists...
In the event of an accident - say a wheels up landing not likely in
Atlas - the FSDO will physically look for your check lists and will
ask you about using them...

Now, on the issue of trusting your GPS.. It's easy, it's fun, it's
fast, it's accurate, and I am just as human, and just as lazy, as
anyone... But I do - on cross country flights, night flights, and of
course when filed IFR - drag out Howie keefe and open it up, dial up
a relevant VOR, and keep a cross check on my position, even while
following the little arrow on the moving map...
I do not do this when bombing around vfr in the local area, I just fly
i.f.r. ( I Follow Roads - and water towers - and railroads, even a
flock of geese now and then ) I do tend to control my altitude though
- old IFR habit...

I think the best way to keep your 'precision' up is to work on a
rating... For you and Mary, this should be the instrument rating - as
that is probably more immediately useful than say, a float plane
rating... But any new rating will keep your mind focused on flying
more precisely...
Or, how about just getting some aerobatic time with an instructor...
Nothing like controlling speed/angle/bank/power/Gees on a constantly
changing flight path to improve the old straight and level...

denny