In order that it will be simple to move around if need be, we place our UM2 on a rubbermaid design cart and there's storage space underneath for material and filament. It turns out the face of the cart wasn't totally level. I really could hear (and then see) the UM2 rocking back and forth as the print head went from one side to the other. So I shot a large slab (2" thick) of high density closed cell foam we had lying around and put it up on that. Then I found the normal "feet" of the UM2 were "sinking" into the foam. So I needed some feet to spread out the load.
Additionally I've heard (read) about the electronic equipment of the UM2 overheating. So the feet are there to enhance air flow below the bottom. If the overheating thing is common, uncommon, or simply an old wives tail, I do not understand, but I figure it can not damage.
Lastly, I used plain jane PLA, well technically it's Type Futura Easyfil, so I do not anticipate the feet to help or hinder shaking by themselves. The 2" thick slab of closed cell foam should likely dampen some shaking... but I 'm no shaking pro.