I used to drop the gear too. Usually if it starts doing little quarter slips you have the gear too high. Most people seem to think that it's because the gear's too low. When actually you can run less throttle and have the gear further down, while having steam at boiler pressure at the full length of the stoke. I think what makes it slip is the steam starting to expand in the steam chest and not having a smooth application of power.
They'll let you know when it's time. Sometimes they'll stop on their own too. Living things they are.
Kevin Bush