I was torn on this too, and I tried a climbing in all that gear and I was a sweaty mess. The thought of trying to add those thick layers once up in the tree just felt like it introduced too many chances to make a mistake. I resolved that if it's cold enough to bust out the bibs and the heavy coat, it's cold enough to hunt from the ground and stay out of the trees. Then I can carry in layers, not be all sweaty, and add those layers as I start to get cold.
Last year I walked in in my long johns, sat in a ground blind for 20 minutes before I started to cool off and started layering up. 2 hours later I got my first deer, 10 feet from where I was sitting.