By calling 811 before you dig or plant plants, you are saving potential health and financial risks.  For a cost of zero dollars, when you call 811, someone will come out and mark your utilities.  

The key is to get the electric, telecommunictions, water, and gas lines marked.  

You can use your best judgement of where you, "think", utilities might be, but what happens when you put a shovel through a 2 phase wire?  Some might rather have that than to have to pay to fix a fiber optic cable.  That can sometimes be $10,000 plus.  It is not out of the question for repairs to get over $100,000 in special circumstances.

One little call to 811 will minimize your safety and liabily risk.  But still be very leary.  I once, "found", a live gas line that didn't have a metal, "marker", wire with it.  We were almost at our depth with an auger, and I thought I hit a root. Normally the auger will blow right through roots, but I happened to check, and sure enough we damaged a gas line.  It wasn't severed, just dinged.  But it wasn't marked, and there wasn't a wire with it, so how would we know?

So on one hand it is essential to do, but could be just a warm and fuzzy feeling.  Meaning there is still the potential to damage, "stuff".  811 will not mark irrigation, pool pipes and wires, electric dog fences, or landscaping lights.  You know that shed where the previous homeowner ran an electical wire and buried it just below the surface?  Beware of THAT! Not to mention the utility marker technician possible wasn't thorough or couldn't find something.

Moral of the story, call 811 for your protection, and USE COMMON SENSE FOR YOUR PROTECTION AS WELL BECAUSE THERE ARE NO GUARANTEES!