There's another side to this apart from the power-on order at the start or end of a computer on/off cycle.
If you're using the DPMS features of X, such as setting the StandbyTime in xorg.conf, then when the inactivity timeout arrives, X shuts off the video signal. The monitor then shuts off the backlight and the power light goes orange, but it's still on. Once there's activity again (keyboard or mouse), the signal is restored and the monitor will come back on. This should be OK because it was never off before the "computer came back on".
Where it gets tricky, it seems, is that the monitor can be configured to turn off completely after 3 minutes of no signal. If you enable that feature (to save more power), and if you want to obey the power-on order rule, then there's a potential problem. You need to turn the monitor back on by hand before doing any activity that would restart the signal. But, you can't be sure of doing this because there can be accidental mouse activity that restarts the signal while the monitor is off, and of course you can't see that it happened! So you could be turning the monitor on after the signal is back. ISTM to avoid this you have to leave the 3-minute power-off feature disabled.
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