I shave the jaw line like this...
I start out shaving with the grain.
Then either with the grain again.
Then if necessary, cross grain,
Then if that doesn't do it, I'll shave against the grain. (which I really don't like to do.) At that point, I'm golden. No blood, no irritation, no nothing. It's all good.
I use a Merkur HD with a Derby. I know they are not that popular here, but, the extra level of gentleness, when you use it on your neck, goes a long way. As always, your mileage may vary. (YMMV)
With DE shaving, "Every day is a new adventure."
- Duggo / Avatar photo credit: "Wssst" from Flickr.