Does anyone know, in Spectre, whether Vgb is checked against vbox?
Or are only Vgs and Vgd checked against vbox?

Should Vgb be checked against vbox, or is some of the gate to body potential dropped across the silicon between the body contact and the point right under the gate?

Generally, the source and drain diffusions are close to, and even underlapping, the gate, so Vgs and Vgd are likely to be the voltage the oxide actually experiences.