Here in MA it's the LAW to call Digsafe. It doesn't matter if you are digging only a few inches. The last ime I had a cable buried they used a ditchwitch that went down about a foot. They like to go down this far b/c some people (self included) like to aerate their lawn, and or till it when necessary.
FWIW, I work in the civil engineering field and I've personally seen high pressure gas lines with less than a foot of cover. For this reason, whenever I am using machinery when digging, I ALWAYS CALL DIGSAFE! There are huge liabilty issues.