To be perfectly honest, if my preboil gravity and volume are on target, I don't even bother checking again postboil.
Same here. I used to check, but my post-boil gravity was always what the pre-boil gravity and volume predicted it to be. I think the pre-boil gravity is the second most important, since that is your best chance to correct the hopping rate if the gravity is off. First is the mash gravity, since that tells you that your mash has fully converted and that your gravity expectation will be met.
Once you know your system, the kettle volume should be a given. You know how much water you put in, you know your dead space, and you know how much grain you used, therefore your absorption. It shouldn't be much of a mystery. I measure the kettle volume just to confirm that it is the volume I expected.