Most don't have the top fastened because it's easier to handle and fit that later. Using a level, find the floor's high point to determine the cabinets' high point. Mark a level line through that point across the wall. That reference line guides all the cabinet tops. Intersect that line with a plumb line down the middle.
Any middle cabinet goes in first.
Draw a line down the center of its back and line it up with the guide line. Transpose pipe locations to the cabinet back and drill pipe holes with a spade bit. Slide the cabinet into place, level the top and secure it to the wall studs with drywall screws. Flush and plumb the other cabinets. Use a wood clamp to tie the front frames together and screw them together.