Sat front row at KB during previews and missed absolutely nothing. The Hirschfeld stage isnt too tall, and if I recall correctly, the stage was exactly at my eye level. May have missed some of the stage floor lighting, but no action whatsoever. Definitely did not miss the boots, either!
I sat front row with a rush ticket and I didn't have a problem. You may miss some of the office stuff that happens on the upper level. But Nothing much really happens up there action wise.