"The very first thing that my instructor said when I took a darkroom workshop was to always finish a roll at 35... "
Good Morning,
Toffle is right, and, as others have indicated, the primary problem with extra frames lies in the contacting--unless, by great good fortune, an obviously worthless shot or two happen to lie at exactly the point where it/they can be clipped out and discarded as the roll is cut into separate strips.
Something no one has mentioned so far: With slide film, squeezing out an extra frame or two can be considered a bonus.
Konical