In town I saw this species get pruned as you would a boxwood, very harshly. Magnolias hate pruning. But the shrubs (they were shrubs) have sent out lots of new growth. Still not good for them, but if they can survive a drastic pruning like that then I would not worry too much if you just are removing a small amount low branches low branches tend to produce the least food for the plant anyways.

I have pruned some 'so a bird could fly through them'. No harm done and the effect is very nice when they bloom in early Spring. Prune the hardest at the bottom and as you go up let it get denser so it has a canopy of sorts.

With deciduous magnolias main thing to avoid is removing branches large enough that cavities might result. In this case "large" might be around one inch thick or more, I suppose.

We planted one in May of 2006. It was probably 5 1/2 feet tall at the time. That fall, our lab (he was just shy of his first birthday) pruned all of the lower branches. I was so upset and was sure it was a goner. I clean-cut all of the twisted and mutilated stubs with some loppers. Here it is two years later and the bottom branches are just about caught up with the upper branches. I don't recommend this type of pruning, but apparently the Star magnolia is pretty resilient...