I taught myself this little trick today. I have already used it a second time--so now it's a "technique" (which I like):
Put a "dimension" label on an edge of a side before you push or pull a face, and you can watch the length of the side change "dynamically" (i.e. as you push/pull). I guess if you're a super-excellent SU-user (like Swingman or Leon) then you model your lengths right the first time and won't need to do this very often (only when you modify an existing model).
Bill