Most experts agree early Spring is the best time. Very early for cool season grasses, and a bit later for warm season grasses.

Some landscape maintenance companies will cut them down in the late fall, in order for their client properties to remain neat through winter, but there are good reasons to leave them up for the winter.

"Winter Interest" is one of the best reasons to leave them up. Most gardeners like the look of vertical beige stands of grass in winter. The collect light snow, and wave in the breeze.

They can also provide food (seeds) for wildlife, as well as habitat material.

Leave them standing, and they can collect leaves and snow, which acts as protective mulch for the crown of the plant through the winter. Just be sure to completely rake out the crowns when you do cut them down.

If you've been cutting them in early spring, and they are flourishing, why change?

I agree with donn with pretty much everything :-)) However, I would cut these grasses back much harder than by 1/3rd each year. IMO, that leaves too much dead material standing and detracts form the new growth. I would recommend cutting them down to about 6"......even lower if possible.