As far as I can tell, REAPER's current implementation of Rubberband is pretty unuseable -- at least for normal short-distance stretching.
Other algorithms such as Élastique work much better, but unfortunately all are glitchy and very sensitive to settings.
Here are some examples of different algorithms and modes stretching a plain sine wave. Stretch markers are at zero crossings, and all slopes are flat. Glitching can easily be seen by the blotches of different colors within the pure red pitch.
Élastique soloist:
Rubberband, standard window length
Rubberband, long window length
Rubberband, short window length