In attempting to answer this question for myself, I have done some testing on my systems and it appears that NAV uses approximately 1-2% of the CPU time when it is active, and McAfee uses about 1% of the CPU time, and they are generally only active when the client (RC5 or SETI) is actually writing results to the disk, so the difference for me was almost negligible.
Even so, if I know for certain that the computer will not be used for an extended time, and it will therefore also not be connected to the Internet or accessing any other potential source of virii, I usually will close the Auto-Protect function, since even another 1% can add up over time... 🙂