Preferred is to make your plan, and then install sprinklers. Modifying an existing in-ground system can be a nightmare.

But if you want to go with your current idea (add as yet undetermined modifications later), absolutely INSIST on receiving an as-built diagram of all lines and valves. (And yes, I'm shouting.)

Agree with Jean's suggestion for a mapping of the lines -- this is invaluable info to have on hand down the road, when you want to dig for any reason.

If it were me, I'd to my best to envision where on the property I planned to add new features -- outbuildings, garden patches or raised beds, anything that'll need ready access to water -- and mention those to the system designer. Even if you're not sure what you'll do for an area, stubbing in a connection is much easier to do at installation time then later.

Another suggestion: ask how they will setup the system for easy draining before winter. I don't know what zone you're in, but if it freezes in the winter, you'll want to make sure you've drained all the lines so you don't end up discovering leaks due to split lines or cracked heads, when you fire up the irrigation system in the spring. This is typically accomplished with either a manual or automatic drain valve, the latter being installed at a low point in the line.

Have fun...