What is the best time of year to cut back ornimental grasses ?

Early spring. Leaving the dead foliage through the winter protects the crown of the grass.

If you have tall grass (5'+) tie the grass with jute twine to keep it neat during the winter. Then in spring it will be easier to cut.

When you cut it back in early March or late February, you want to cut it back to about 12 inches.

Medium height grass can be cut back to 8 inches. And for short grass, you can cut back to 2 inches.

the time is not important unless you wait too long... you must cut back before the new growth has begun..... if you cut the new growth, the plant will look raggedy....

http://www.bluestem.ca/april-03.htm

if you like a well kept garden look, the off in fall.... if you like the look of the grasses standing in the snow, then spring is fine....