If it's a metal frame with a metal seat post, then a light coating of grease where the post is inserted into the frame is a good thing. It keeps the post from sticking in the frame. If you have metal on metal, you might not be able to remove or adjust the seat post after a couple of years. However you don't need any grease on the exposed portion of the seat post. Just wipe it off with a rag.

If either the frame or seat post is carbon fiber, don't use grease on it. Get something CF specific like Tacx Carbon Prep.