I've done it both ways. With tearing, you definitely want to be certain that you have a few inches more than you think you need.
And when I tried tearing flannel it was an unmitigated disaster.
Tearing can certainly get the fabric closer to grain than it sometimes is when it is cut from the bolt - so is sometimes useful in figuring out where the wonky is. I would never use it for my narrow strips, just for the big pieces.
Mostly, if I've got a large piece of fabric (I like shopping the end of bolt sale for stash building), I'll tear off what I need rather than wrestle with the whole 2-3 yard piece on my pressing table.
Cheers, K