Good to knowEdit: He's correct, if he has to go northwest from Fontana he will be in fairly high country, and 10K is not unreasonable to represent a safety margin. My previous reply was inaccurate if anyone read it in the 27 seconds it was up
Edit 2: Going through Cajon Pass, in a lightly loaded airplane, at the wrong time of day, or in certain weather conditions, is not advisable IMHO. Going up and over would be a better choice some percentage of the time. The boundary between the LA basin's moist cool airmass and the warmer drier desert airmass creates very significant localized instability and turbulence. When it bulges out into the Mojave desert that boundary turbulence is called the "El Mirage Shear Line", and we crazy old sailplane drivers would ride that instability for hours on end. Dealing with it at low altitude in the Cajon Pass, if you're not a sailplane pilot, might be an unpleasant experience. Early morning before all of the air starts to get active, probably not a big deal.