Checking for underground utilities

I'm about to do some passive bed preparation in the front yard in advance of planting trees. (This is basically composting in place, and I'll write a post on that.)
I'm trying to comply with as many codes and safety precautions as possible, so this morning I created a Call Before you Dig ticket, which alerts you of underground utility pipes and wires. It may be overkill, but I will rest easy knowing I'm not planting trees over utilities. Some utilities have sent emails, others have marked up the lawn with spray paint and flags.
There is a gas line running through the yard right by the steps. Good thing I found that out! It really changed my plans, because I do not want to plant any trees or shrubs near that area now. 
I happened to be home when another Call Before You Dig utility worker came by and he chatted with me about it a bit. Gas lines are supposed to be 18 inches below the surface, but he says they heave up sometimes. He said be very careful digging within a foot and a half to either side of the gas flags, even if I am merely using a shovel. 
I also called the zoning office in our town a few times. I've gotten somewhat confusing and conflicting responses, but essentially they told me not to block views of turning cars. (We live on a corner lot.)
I called the health department to ask them about the septic tank and field. More info on that in another post.
So I think I am good for bureaucracy. Almost ready for digging!