New Sprinkler Installation – Rototill/Level Before or After? Any other “Pre-Sprinkler” Preparation

I have a (mostly) dirt and weeds backyard to which I am planning to add sprinkling and new sod next month. However, my locality requires (and I agree since the soil is poor) that I rototill in compost to prepare the soil before sod is installed. My backyard is also uneven and bumpy. Thus, would it be better to rototill/flatten the yard before or after I begin digging the trenches and laying pipe? Is there any other prep I should do before installing sprinkling?