Most makers of carbon seat posts and carbon frames list very precise torque limits for tightening the seat post bolts on their web sites. But, I've never seen a bike shop use a torque wrench when installing a seat post.

With my own carbon frame, I use a very small compact "pocket" wrench to tighten the seat bolt. That makes it difficult to over-tighten the bolt. I just get the bolt snug, and try to twist the saddle left and right using moderate force. If the saddle stays put, I test ride the bike and test for slipping. If necessary, I tighten a touch more, and do another test ride.

I think I could get over cracking a carbon seat post, but if I ever cracked the frame...that would take awhile to get over...