Q.Magnolia about ten years old is leaning slightly
Trunk 6″ wide at ground but 4 inches just 4-5 inches above that. At about 36 inches high it is about three inches off vertical, but at about 5 feet the trunk narrows to about 3 to 3 1/2 inches and bends about the same amount more in the same direction. I put a 2 inch wide tie-down strap around it about a foot above that bend with the anchor end at the base of a t-post driven into the ground at 30 degree angle. I applied pressure on it, intending over time to increase the ratchet a click or 2 periodically. Will I damage the tree internally doing this. Tree is 15-18 feet tall.

Thanks for your assessment. This is a replacement tree for one of about 20 we lost with Hurricane Rita, and it is a beautifully symmetrical tree at the entrance to our home. There are several gorgeous huge magnolias here in town, and the lean on this one just aggravates my wife's and my senses...much like if one our porch columns was leaning. I understand it probably isn't necessary and we're probably a bit anal, but we really do want to straighten it if it's not going to damage it. Thank you so much for your input.

The method that you describe to attempt straightening the tree trunk is conservative in respect to the gradual pressure/pull and is an intelligent approach. If you don't get anxious and apply too much pressure at one time it should not over stress the woody structure internally.
But I question if it is really necessary to straighten it. Sometimes an asymmetrical balance is okay. As long as the structural integrity is intact, no uprooting or defects prone to breakage, good weight distribution can be achieved with selective, directional pruning of the branch structure, without straightening the trunk. Think Bonsai or rather Niwa-ki concept of Japanese style garden tree where bends and twists in the structure are encouraged and add an artistic, unique character.