Your first mowing after a new lawn installation should be after the sod has had time to knit with the soil. Make sure to mow diagonally to the way the sod was laid with an initial 2 1/4" height on the mower.
Not just the appearance, but of more importance, the general health and vigor of the lawn is contingent on a good mowing routine. Only a sharp, properly adjusted mower set to a minimum of 2 1/4" in height should be used.
Your first mowing should be after the sod has had time to knit with the soil. During rapid growing times it may be necessary to let the sod dry enough to permit mowing during the 10 to 12 day period. The first few mowings should be done with a rotary mower diagonal to the way the sod is laid. Subsequent mowing should be done whenever the grass clippings measure approximately 1/2" or 25% of the grass plant.
Never cut off more than 1/3 of the plant height (approximately 3/4") at any one mowing. Removal of a greater amount of the plant may result in shock, unsightly appearance or susceptibility to disease.