Everybody will tell you there is no benefit to using used water, but I put not only the water but pre-cycled rock, bio balls, sand, and any other media I can, into every build I do.
I recently started a 70ish gallon system-(46 bow with 29 sump)- 2 cups of sand, bio balls, 2 pieces of LR, and 5 gallons of my water= -0- cycle time, no ammonia, no nitrites, no nitrates.
2 weeks later it is stocked, and running with 20+ corals and 8 fish.