The school I learnt to fly at didn't lean aircraft at GFPT level. It was introduced at PPL level and a matter of course for CPL level.
Climb fuel was never calculated as a more conservative company fuel burn figure was employed for the duration of the flight.
In the grand scheme of things a few litres here or there is no big deal. Flight training just teaches you to be safe. Operational efficiency comes through employment, and you fly the ship the way the boss wants you too. Of course every time you move to a different operator, the game changes. Even when operating the same type. Be adaptable.
In the real world of aviation we try to keep things as simple and as easy as possible. Pilots love to overcomplicate things, particularly the airline wannabes and hopefuls or the crusties high and mighty on their lofty perch, preaching superiority over their brainwashed flock. If you just work on a solid basic foundation in your flying and preparation, the battle is won. The engine is not going to fail because you didn't lean it. It might fail in time if you didn't lean it properly.
You're not going to run out of fuel because you didn't account for climb fuel on top of your cruise burn in a light aircraft. You might overlook something in your planning because you wasted time and head space on it. You are not going to destroy a small engine running it rich in the training area. You might have a mid-air collision with another aircraft because you were not paying attention to where you were going.
Keep it simple!