Our area has gone from drilled to 15's to 30's, with no or little yield loss. With same yield, producers factor in the cost of a splitter vs 30" planter and the reduced seed cost. Also, in the recent years - but not this year - better resistance to white mold with the 30's. 

Most with 30's are planting 140 -144K population. I've experimented(okay, screwed up) and planted 121K with no yield loss. I prefer the 140K to promote consistent emergence. Anything more, and I'm just growing more straw and wasting seed.