UV editor calls both, so why don't other editors do the same? Especially for the timeline it would be important, since there's no other way to find out the scroll position and zoom, thus it's impossible to overlay anything aligned to the frame steps.
http://blenderartists.org/forum/showthread.php?283148-Draw-over-a-timeline-region-in-respect-to-its-frame-numbers
Description
Description
Event Timeline
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Fix in svn, thanks for the report.
The following editors now support PRE_VIEW and POST_VIEW: timeline, graph, action, NLA, sequencer, image, clip, 3d view.
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Wow, thanks for this quick fix, which was kind of a feature request! Scripters will much appreciate the callbacks for the timeline in particular, allowing all kinds of useful overlays!