Be careful on this. As in the real estate industry, placing type of treatment is all about location, location, location. Reflecting high frequencies in line with the center channel is not the best approach considering most voices are near or higher than 500Hz. We are not interested in that because we do not want interference with speech intelligibility. As with anything, cookie cutter approach does not work or apply here. The best thing to do is know what frequencies are bouncing where, understand what affect that has on the listeners, and is it desirable or not. Generally speaking, speech intelligibility is a pretty important thing in your theater.