You can safely polish the exhaust bridge if it gets scoured from a minor seizure, using a "red stone". That is a small rectangular stone block of fine grit and soft material, they are commonly sold at industrial supply stores. They're used to clean and sharpen lathe tool bits and sold in different grit/colors. Red and green are the finest and softest. Use the stone to buff up and down the length of the bridge until its smooth.
All OEM cylinders have relieved bridges that are machined with CNC grinders, but aftermarket plated cylinders are hit or miss. I use a three stage process on my cylinders, starting with a soft stone, then a fine sanding roll, and finishing with a Cratex 120 grit rubberized abrasive.
Some 125 cylinders like the YZ, TM, and CR have such narrow bridges that they tend to wear a notch in the piston and ring. Relieving the bridge more just makes it worse. The solution is to either replace the ring often (4-5hrs.) or have the bridge welded wider during the cylinder replating process. My company and RPM offer this service and fitting of the exhaust valves is required to accomodate the weldment.