I always add yeast nutrients (with the exception of Go-Ferm) after I pitch, and the bulk of the addition happens once lag is over (i.e. once you can see signs of active fermentation). That way, any inorganic nitrogen (DAP) in the nutrient won't stress newly hydrated yeast cells until they're ready for it, and also you don't end up adding nutrients that could benefit spoilage organisms that may have taken hold before your yeast colony grows.