I second johnstoeckel.
I don't yank the thread either at the beginning or the end of a fly for fear of bad things happening, like a bent hook, causing a break under the whip-finish wraps so the whip-finish is no good, etc.
I also tie my whip finish by hand, but I learned quickly from the Bass Pro Shop video on fly-tying with Lefty Kreh. It took me about 10 minutes to figure it out, and it's super easy. I usually do a double whip-finish in case 1) one of the two is no good or 2) one of the two gets torn apart by many hungry fish. This makes me feel much better about omitting head cement, which can turn fish off due to the smell. Really small flies may only get a single whip-finish to prevent too much thread build-up. In that case, I take my chances.
And wherever the river goes, every living creature that swarms will live, and there will be very many fish. Ezekiel 47:9