Mulching with grass cuttings

Grass clipping will rob nitrogen from the soil as it breaks down. Alfalfa is the best mulch with great nitrogen/carbon ratios that will not rob anything from the soil as it breaks down. Grass clipings can be a great place for mould too. If these clippings are from your gasoline fuelled lawn mower, they will be polluted and not healthy for your plants either.

Regardless of what it's made from, you should really give mulch/yard debris a season or two before working it into your potting mix. Using fresher stuff is going to cook your roots as it begins to compost and probably lead to nitrogen spikes and pH fluctuations later in your grow.