I generally wait until I've attached the bows to each pew before I cut the tails, although you can do it ahead of time if you wish. I wait because I like to cut them so the tails rest just an inch or so above the aisle floor.
Just be sure to have good, sharp shears so you leave a clean cut. Dull scissors can make a mess of tulle with jagged edges.
If you wish a "V" cut, fold the ribbon over and cut a sharp diagonal cut that points upwards towards the center of the ribbon.