If you're referring to light acclimation time for the inhabitants of the tank, I can tell you I didn't do it when I replaced my PCs and didn't have any problems. I'm not saying I did it right, but everythings OK.
yup me to. we added at 175 MH and just fliped it on with no problem at all. seemed to have no effect. like said above it may not be the right way to do it but i saw no ill effect