What should these be set to? What determines these settings?

If you are using a simple setup with an audio interface and a computer the usual recommendation is to set the timing master to the audio interface to use the interface clock. Computer clocks are not generally as accurate or as relevant to the recording process. If you are trying to synch a bunch of external gear that is sometimes more complicated.
 
Edit/Preferences/Audio-Devices

Thanks for the reply! Simple setup. Focusrite Saffire Pro 40 and Focusrite Octapre, connected through ADAT connection into computer. It gives me the option to set any of my interface inputs as timing masters. Does it matter which ones I choose and, should both recording and playback timing masters be set to the same, or different interface inputs?

Bump
Never got an answer to this.

It doesn't matter which.  Set the timing for Playback to an output channel and the timing for Record to one of the input channels.  I use the first available for each.