Grass clippings have been the "almost perfect' match for all the leaves I collect in the fall. Ample supply in the neighborhood, and they mix so well together. Yes I do get grass and weed seeds, but I get acorns, walnuts, elm seeds etc. in the leaves, but the composting kills the great majority . I think even if there were zero weed and grass seeds left in the compost, by the time it is spread out in the garden it becomes contaminated by wind blown and water carried seeds. My solution is to mulch with shredded leaves over the compost spread in the garden. That pretty much blocks the emerging weeds.
If you think that grass clippings belong on the lawn, then leave yours there and pick up the neighbors who believe they belong in a plastic bag. I pick them up by the truck load.
But for an easy solution, high nitrogen chemical fertilizer is easy to work with, has no grass or weed seeds, and can be measured out in exact quantities, and will certainly heat up a compost pile.
