Being able to use a jibsheet for something else in an emergency -- like a towline -- would make me tend towards tying them to the clew individually.
If you''re in a situation where you want to change
jibs, it gives you a sheet you can use for the new sail before you lower the old one. A cow hitch will work, but it limits your options, as capttb notes. Tying bowlines is good a good way to practice speedy knot-tying or to show off for guests you wish to impress.