Horatio,
Who ever said anything about not leaning the mixture enroute?
I said at a GFPT level burning around in the training area while learning to fly the aeroplane, i.e stick and rudder suff, leaning the mixture is not that important.
Just as when learning to navigate, it's not that important to account for climb fuel in an aeroplane that is burning 24 L/ph at best, and 44 L/ph at worst (depending on type).
Let's use your 16 minute example on a 'heavy single'. 60 L/ph cruise and 75 L/ph climb.
16 minutes at 75 L/ph = 20L
16 minutes at 60 L/ph = 16L
We are talking 4L difference to get to 8000ish feet if your performance is 500 ft/pm ROC.
On something like a 172 we are probably talking 2 litres difference.
I personally think the time (in your allotted planning time) is better spent reading NOTAMS, studying Australia's completely and utterly ridiculous airspace, weather, suitable, adequate and emergency landing points etc etc etc.
PS the aeroplane doesn't know if it's dark, over water, or flying IFR!