leigh,
I think my '89 325i calls for timing belt about every 60-65000 miles. I don't know why your '88 would need it more frequently. I wouldn't be comfortable with a shop doing anything without my ok first. You are the customer, you should call the shots. Frequently it is water pump replacement time about the same time as the timing belt, but, again probably more in the 60-65,000 mile area. The labor cost is theoretically decreased if a mechanic only has to go in once to do both jobs.