I am inexperienced with medium format - just got a camera - and due to the large mirror size it is advisable to use the mirror lock up function at higher speeds than you would normally do in a 35 mm camera.
I read somewhere should be at 1/60s and slower. I ask what is your personal experience about this. Please post only your experience and not something you read on the web.
My experience has been that it pays to lockup the mirror at any speed when sharpness is paramount, especially when using a poorly damped tripod.