replace the little spring washer under the dial when re-assembling a watch after cleaning/oiling? I have attend two seminars where the instructor said they are not needed, but wonder why they were installed orginally? Thanks

I usually put the washer back (either the original or a new one). It is a tension washer intended to keep the hour wheel and hand from slipping up the cannon pinion. I see two purposes to this. One is to be sure the teeth on the hour wheel fully mesh with the minute wheel at all times. The need for this will depend on how closely the dial rests on the pillar plate. Another purpose is served where there are close tolerances in the hand clearances and you want to be sure the hour hand will not hit the minute hand. This may not be too critical on a big 18-size watch but can be important on a thinner watch.