You could cut out the flowering stems once they are done...not sure about shaping the foliage. Different Knips have different foliage. If you are unhappy with the plant's appearance, I don't think it would hurt to experiment one year but plants with swordshaped foliage might not look so nice with the tips cut back. They will get a brown edge to them.

Here is a quote from Paghat's Garden about pruning Kniphofia:
http://www.paghat.com/kniphofia_pfitzer.html

"The only other care it needs is yearly "under-pruning" as the old saber-leaves dry out & are mashed down by new growth, so that eventually the under-edges of the clump can be unsightly. If the clump becomes overall too floppy, it can be sheered very short, & will pop back tidily."

So it looks like you can from their experience.