So far I haven't had an overly stuck seat post. The Colnago Seatpost has had mighty few adjustments over the last 30+ years.

Personally, I'd leave it uncut. The next person might want it longer, or perhaps you'll loan the bike out sometime. Or, maybe tinker with the pedals, cranks, or other adjustments that would affect the seat height.

The more post, the less torque on the end of the seat tube & clamp.

Some MTBs have mighty long seat posts.