If you've seeded some bare spots or a whole lawn, hold off on mowing the area until the new grass is between 3 and 3 ½ inches long. Make sure your mower blade is nice and sharp, and then cut your grass only when it's dry. You also want to avoid mowing too short. (You only need to remove 1/3 of the height.)
If you've seeded over your existing lawn, you can mow as needed, but try to cut back the frequency to limit traffic on the new seedlings.