I don't know how many bikes I have seen that I cannot get into a repair stand because the seatpost is stuck. Yes, grease the seatpost, grease the stem, grease the water bottle cage braze-ons, grease the rack braze-ons.

Greasing the seatpost won't make it slip. I recommend greasing the seat tube and steerer tube once a year.

Don't grease carbon fiber seatpost!