I can't speak to how the Roku handles it, but can give some technical details of different framerates. I'm a broadcast engineer and deal with them a lot.
In the US, the framerates for TV are 29.97 and 59.94. For SD it's 29.97, for HD it could be either but usually 59.94. (I.E. 720p/59.94) These are the framerates supported by the actual TV set, not the Roku or the m4v. 30fps and 60fps also work because they are close enough, and in reality might even be 29.97 or 59.94 depending on the device feeding the TV. Any frame rate other than these needs to be converted somehow to one of these to display on the TV. The Roku may do this, but I can't speak for that.
15 fps is easy to do because you just display each frame twice. So are other multiples, and you may now see TV's that brag about being 120Hz-though I doubt any content is readily available at that frame rate.
24fps (or 23.97) is film. It is harder to convert to 29.97 or 59.94 because some frames will need to be displayed twice, others once. This is called "pulldown" and there are several schemes.
25fps and 50fps are PAL and should be used to target countries that use the PAL TV standard.
Again, I can't speak for how the Roku deals with it(IE converts it or passes it as is), but I would avoid anything other then 29.97 or 59.94. It is possible that you may have a set connected via HDMI that supports an odd framerate, but someone else might have set connected via the component output that does not.
