If the speeds are reliable and consistent, you can always compensate - although that's inconvenient. However, if the lubricants are deteriorating and there's dirt inside, you might have a shutter that will continue to change the speeds. Typically a proper CLA will result in all but the one or two highest speeds being very close to the marked speeds, and very consistent.
Originally Posted by k_jupiter
I take a change in speeds as an indication that the shutter needs servicing, the same as a clock or watch changing rate is an indication that it's time for a cleaning.