The idea of taking off with neutral trim is so that as the aeroplane becomes airborn you can feel the pitch forces and verify that they are as expected. You will soon get to know what it should feel like 2-up, 1-up, etc and once you are established in the climb, with the expected amount of back pull on the yolk and it all feels as it should, you can trim off the back pressure for the climb
It's just good practice. You can't accurately trim a light aircraft unless you can feel the forces you trying to trim-out. You can only do that once you are flying. If you 'guess' at a trim setting before you set off, it might be miles out in the air. In practice, once you know an aeroplane, you might give the trimmer a bit of a tweak either side of neutral (I do in the Chippy - a tad more nose-down if I've got someone in the back). But I still re-trim once climbing.
The same does not apply in large transport aeroplanes because they have a much wider CG range and their tailplane setting is therefore pre-calculated for the actual CG.