Depends on how long you plan to trench, and if there are other elements like massive tree roots that make manual digging difficult, i would suggest you not to use machine trencher to be on safe side. Just pay cheap labor to manual dig, and ask them if they touch something hard (like a pipe), stop and check.
Another thing to prepare before digging is to call 811 free utilities line mark service. They will send people to mark power, cable, gas line in your backyard. Then you know where utilies enter the house and avoid digging in those area.
