Puttinggreens has very good advice. I wouldn't bother to spray Roundup a second time. Crabgrass(and all summer annual weeds), if you missed any, will die at the first frost--no problem. Since Roundup will not kill crabgrass seed--you will need to apply crabgrass control late March anyway, and again 6 weeks later, to be sure. Both aeration and roto-tilling are a risk to the sprinklers--mark the heads carefully. Bury the wires and tubeing deep. Rototilling makes a mess--you get a lot of clumps of sod to rake out, and haul away, (need dump truck). Puttinggreen is right--use aeration.
Hurry--you want to sow your seed 6 weeks before frost. Seed in fall--not in spring. (Unless you are seeding Bermuda).
Remember Roundup is absorbed and inactivated by soil--no waiting period.