Some cold season grasses will respond to in-season shearing by sending up new growth. Carex muskingumensis and Festuca glauca are among them.

Warm season grasses, however, like S. scoparium, are finished growing new shoots until next year. The existing shoots will elongate until the plant starts to flower, and if you trim the blades, they will have squared-off blades which will brown at the edges.

My experience is that bluestem will put on a push in its growth in late August and september into its October bloom cycle. I seems like if you trim it a bit, it will have time before it is ready to push out its blossom.