This project demonstrates that Reaper 5.973 is not passing midi message sample offset on input in OSX. This results in unusable jitter. I haven't yet run it on Windows, but my ears tell me there's jitter there as well.
https://drive.google.com/open?id=13B...OvO82i0MJm5c1n
I discovered this while trying to send sequenced MIDI from Sonic Pi. No joke, all this time I thought my timing just sucked.
While researching the issue, I came across this article by Geert Bevin, which concluded that while many DAWs had this issue, Reaper 4.31 did not. I installed Reaper 4.31, and he is correct. I installed a fresh portable version of the latest version as a sanity check -- there is a definite regression.
http://expressiveness.org/2012/12/04/midi-jitter