Always, ALWAYS run a tracer wire with anything new you put in the ground. You never know when you may have a phone line, cable line, etc go bad, get struck by lightening, get cut, etc...and need replacing. When replacing other utilities, you can easily end up with a major problem when a plow blade goes in the ground IF the installation contractor can't get an exact location on your sprinkler system.
You also need to be sure to have all of your utilities located before putting a trencher in the ground. Generally gas and power companies will locate all the way to your house(to the meter, generally on your house), but water only locates to the meter(generally near the road, near the main), and phone/cable companies generally only locate to a ped/pole near your house(only locate the main cable, leaving you personally responsible for your own). It's imperative that you locate this stuff beforehand and take precaution to avoid cutting these utilities. The bill for a mistake in cutting one of these utilities can get quite expensive, even on occasions when the repair is actually quite simple. Oftentimes, an ATT tech will require a brand new phone line be placed from the PED to house(especially in the u-verse era), rather than a very simple ground splicing. This requires an installation contractor come out and place the wire(minimum depth 8-12" if subcontracted to anyone outside of att itself), which in turn, has potential to interfere with the freshly laid sprinkler system, and the rest of your underground utilities.
Any questions, let me know... spent ~ 1.5yrs as a construction foreman overseeing ATT underground installations on private property for the southwest quadrant of the state. It's nothing "difficult" about what you are wanting to do, but it can be tedious at times, and overlooking one little detail can turn it into a nightmare, QUICKLY...