Comment 2 for bug 525508

Only affects rotating by keyboard shortcuts ('[', ']') but not when 'Ctrl+dragging' the rotation handles with the mouse, which uses the same preference setting: «Constrain a rotation or skew to a multiple of the Rotation snap angle.»
<http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Transforms.html#Transforms-Mouse-SRS-RotatingSkewing>

(tested with 0.48+devel r9931 on OS X 10.5.8)