Cut Seatpost to save weight?

A lot of the seat posts have a mark on them to indicate minimum insertion (or I guess this could be viewed as maxim height too.) So you don't want to exceed that. Regarding cutting it's pretty easy you can get a blade for a hack saw that will do the job, you want more of file surface on the blade over a fine tooth (if that makes sense, when you looking at the hardware store it will). An easy way to make your cut straight is to use a metal hose clamp. You put the hose clamp on the post, tighten it and use that as a guide for the cut. There is specific carbon paste that should use on the post too.

Most carbon seat posts don't weigh that much, why not get one that suits, use the paste and leave the length alone? That way you can re-use it on an other bike or more easily sell it down the road.

Not to mention some of the lighter posts (that are actually strong enough to last) are aluminum.

Are you a weight weenie going for the lightest bike possible?  Then yes, cut a post to just what you need.  Otherwise, you are wasting your time.

(And yes, my road bike is a project bike and I have a Thomson Masterpiece post that is cut down in it.  But the bike also only weighs 14.1 lbs. and is a size 58 with clincher wheels!)