You should never tilt your seat

"Yes, if you get a chance you should sand the pads down before hitting the track to smooth out their surface and eliminate the layer of grime that forms where they rub against the discs. This is actually why there are bores drilled in the discs, but of course their cleaning action is limited by their size (and it they were any bigger the disc would break easily). The latest disc models are made with slotted rather than bored surfaces for precisely this reason: improved cleaning with reduced risk of cracking."