I've had 2" of composted manure added to my desert front yard (New Mexico, zone 7a) to serve as new topsoil after having someone with a bobcat and a dump truck to haul off the gravel and plastic sheeting that was previously there. I'm planning to plant Buffalo grass, Curly Mesquite grass, or some kind of prairie grass mix this spring, but before I do that, should I rototill the compost into the existing soil? Or just plant right in the compost itself? I worry that if I rototill, ancient weed seeds that were covered by the plastic and gravel for decades will be brought to the surface, but if I don't, the grass roots won't see any need to spread deeper than the 2" of compost, inhibiting its drought resistance. The existing soil is heavy clay.
You need to dig it into the soil, by rototilling if that's your choice. If you don't, you'll have a nutrient, humus rich layer on top of solid clay, which will act as a pan and create a drainage problem. If you weren't planting in spring, or were able to add the compost to the soil surface last autumn, you could have left it spread out and it would have been incorporated (to some extent) by bio activity into the layer beneath, but with only a short time between now and wanting to plant, you will have to do the work yourself.
There is a risk of unwanted seed germination, but depending on how the composted material was produced (anaerobic or aerobic), there may be viable seeds within the added material itself, regardless of any which may be present in the original soil, which would mean you would get this problem whether you dig or not.
Once turned into the soil, you'll need to let it settle for a week or two, then level the area for planting any type of grass, making sure to even out any soft spots, troughs or hollows.
UPDATE: I'd still turn it over, but it's up to you - you haven't said whether you're sowing seed or planting actual plants, or laying sod though. If you're sowing seed, you need to rototill, planting grass plants individually might be okay without rototilling, laying sod, rototill. Sorry...!