The valve seats should be inspected in the same manner and at the same time as the valves. If the valve seats need to be replaced, refer the job to a reputable machine shop. Seat replacement involves heating the head to as much as 300° (150°) and chilling the seats to as low as -240° (-150°), not a job for the do-it-yourself. The seat would also need to be machined after installation. Bmw valves and valve seats tend to be very rugged and long wearing. Usually a valve job will return the seats to usable condition.