Do I need to encode video in both ogg and webm? I'm doing some HTML5 video work; I'm taking uploaded video and using ffmpeg and related packages to convert the original videos (usually .mov, but whatever) into mp4, ogg and webm versions for presentation via video.js.  The ogg and webm encodings are both working nicely in Firefox across Windows, Mac, and Linux (the main reason for producing both, it seems); my question is whether, as a practical matter, I need to produce both ogg and webm versions.  Both seem to be working well; I'm asking out of an interest in not producing stuff that I don't really need.  Any thoughts?  Thanks!
Probably better suited for Stackoverflow but I think its still a valid question for this SE.

Producing theora(ogg) versions of your videos is actually redundant. Any browser version supporting ogg is also supporting webm.

For questions like these I recommend using caniuse.com.
If you compare webm with ogg you quickly see that you cover the same browser versions and more with webm.
Unless you need to cover Firefox 3.5+.