Is it better to slash long winter grass before grazing cattle?

Not if you want them to do some stockpile grazing. Stockpile grazing is great to use during the early parts of spring when the grasses haven't quite come up yet. It helps put more manure on the pasture for the grasses to use, plus it eliminates most of the cover that might hinder the grass coming up. Of course, this all depends on where you live. If you are in an area where you don't get much snow--if any--and have long, dead, winter grass in the pasture you need to get rid of, you could mow it a bit, or graze your cows on it and let the cows get rid of it for you.