The current 'cool thing' for park skiers (or anyone who spends significant time skiing switch) is to center mount their ski. This means the middle of the boot is at the middle of the ski, in the very center... that's where 'center mount' comes from. This makes the ski feel very balanced in the air while spinning and should feel more natural going backward (tip and tail are the same length so the ski feels similar forward or switch).
When people mention '3cm back' they mean 3cm back from true center.
The best way to measure where your current binding is;
1. Inspect your boot, on the side of the boot there should be a mark that corresponds to the center of the boot.
2. Place boot in binding, find the center mark, make a mark on the ski at this point.
3. take a tape measure, run it from tip to tail following the topsheet of the ski. You are bending the tape down the shovel and up the tail.
4. Divide the length by 2.
5. Use the same proceedure to measure to the center of the ski.
The difference between where 'true center' is and where your boot center line up is how far back from center you are.
If you like to spin, butter and ride switch then a center mount is the way to go. If you ski all mountain, soft snow, etc. then I'd keep the ski in a more 'traditional mount'.
Re-mounting the bindings is no big deal at all.