I did put an adverse camber on my big helix. I did have an issue with my class 103 electric (a CoCo), that despite being heavy, and with a good pulling power, it couldn't haul anything (literally, nothing) up the helix. I think the adverse camber was causing the loco to just rise up onto only the middle axles of the two bogies, causing a loss of pulling power.
But on the shorter spiral I didn't put any shims in leaving the track flat and it has no problems going up.
I am as yet undecided whether to only run the 103 in one direction, or to try to pull out all the glued shims over 4 1/2 loops of helix so I can run it in both directions.
We live and learn!