As mentioned previously, if the MK is out of balance some motors are going to have to work harder than others, in addition you will have less authority in a given axis due to the fact that the motor is already putting out more power than should be required for given task, similar to having a twisted motor .. (already using certain amount of yaw to compensate)  will have more yaw authority in one direction than the other.

Balance is very important if you are trying to build in some level of fail-safe. If the craft is out of balance, possible failure could be on heavy portion of MK .. and as such will have substantially more difficulty maintaining stability.

A quick check to verify that the center of gravity is good is by lifting up the MK under opposite arms with ink pens/pencils (use as a pivot point) and use your battery to get the CG right. In my case I used two of the stanchions that I built for the ACC calibration to balance the MK and check the CG.

Center of Gravity Flight Test
If you have a ZigXbee you can select the FC in the MKtool during flight then hit the 3D button and you will get the Balance tool graph. This is a great way to test the Center of Gravity position. The red dot points to the center of gravity and should be close to center. Since your Lipo is the heaviest item on the MK, move it or other stuff around to move the dot in the center.

Note: Check the ACC setting on the Balance screen in the MK tools it should show a green rectangle in the center of the screen a red one and it is not calibrated correctly.

Other Vibration Correction

While MK's with larger motors tend to create additional vibration, the following simple checks can be taken to eliminate vibration sources that may crop up even in the standards MK's.