I think MC controls that for you if you set 'Enable Automatic Display Settings changing' under Tools, Options, Video.
I have this enabled with the following settings:
Film (23.976 fps): 23Hz
Film (24 fps): 24Hz
NTSC (29.97/59.94 fps): 59Hz
PAL (25/50 fps): 50Hz
Default: Desktop settings.
When I pop up the MadVR stats (control-j, control-r to reset stats) it shows the correct refresh rates to match the source (23Hz becomes ~23.9756 with 1 framedrop every 2 days or so). Initially I rounded the fps up and chose the nearest Hz to match, but that doesn't work for me. When I set 23.975 fps to 24hz it literally sets it to 24hz and starts duplicating frames more than it would drop a frame on 23hz. This may be different for other videocards/drivers, I don't know.
What I don't know is if it there is anything to gain by setting rendering settings under Rendering/General. Things like full screen exclusive mode, use separate device for presentation and use D3D11 for presentation. And then there are the exclusive mode settings to deal with.
Generally I leave all those as they are as I'm not sure what it does and if it would improve anything.
