I am using a Manfrotto 3-way head so it is very easy to adjust and generally speaking, while the bubble levels in the head are fairly accurate, I don't rely on them. I was more uncertain if I should trust a spirit level resting on the camera or image projected on the ground glass.
David, I also never considered whether the ground glass is straight when it was installed - something for me to check tonight.
Clive, I agree the aesthetic is more important than the technical but flaws in technique detract from the aesthetic. I know when I have a good image, I want to make sure I am capturing the image as I see it so I don't have more work than necessary in the darkroom.
Really enjoying this discussion. Thanks all!