From all the PhDs I've looked at and from my, half-written one that I am formatting as I go, the main chapter titles are centered and all the sub-ones are on the left, flush with the left margin. I think the main thing is to be consistent whatever style you choose. My uni does have a formattting guide and there it says to center the main title and all others to be on the left.
Best of luck submitting, you lucky thing :-)