If you do leave enough of the lower stems unpruned so the vines can reach the 1st branches in the trees, then over time the lower portions of the vines left unpruned with get brown and thick with little to no leaves on them. If that is okay with you, you can do as you say. If not, then you need to prune them back closer to the ground.

Another option to get the vines into the trees is to use some sort of bamboo stick or similar item that you will put into the ground near the clematis and oriented towards the branches of the trees and the clematis can climb that into the tree.

Just realize any clematis planted near a tree is going to need supplemental water and fertilizer since the trees will be competing with the clematis for both items.

It's amazing what can happen in your garden over time without you realizing it. I'm not a clematis fanatic. Don't collect, don't plan where to put what, don't water, prune, or fertilize them. Just plant and forget. While pruning I counted and I found 29! There are others that I KNOW are there but can't find. And there are lots of other places where I need them!