Answer: First off, if you hired a landscape contractor to do the seeding, I'd check with him/her to see what they recommend.
From my experience doing landscaping work, the time to roll a new lawn is before and immediately after sowing the seed. We always spent quite a while raking the area, adding topsoil as needed to fill in low spots, and scraping high spots. We used a broad landscape rake for this (or a tractor-pulled york rake). We then rolled the area, adding topsoil as needed--before sowing the seed. Once it was graded to our satisfaction, we spread the seed, then rolled again (to ensure contact between seed and soil). The key is to make sure the soil is adequately firmed down (not overly compacted though) before planting.
If your grass is already growing, I'm not sure rolling is the best tactic. If the seedlings are young and not well established, rolling could damage them and set them back. And I would never roll wet soil--you don't want to compact the soil too much. Moist soil is OK.
I've heard of people rolling their lawn in the spring, to firm down frost heaves, mole damage, etc. If you've been riding a tractor mower over the area, then I would think the tires would have compacted the soil more than the roller would, so I don't know if it will help in your situation. I suppose you could try a small area and see.Try filling the roller only 1/3 full at first.
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