I also use a plumb line and do so while holding the rifle against my shoulder. I've used levels before (with and without a line), but the scope always looks canted when I'm holding the rifle due to my grip/shoulder pocket.

I'd rather have the scope level to my body than to the rifle. Forcing myself to adjust my grip is, in my opinion, worse. The barrel doesn't care if it is rotated about the bore slightly. This is more repeatable for me.