I just heard a You Bet Your Garden podcast on coffee grounds. I believe Mike McGrath says to wait until after acid-loving ornamentals bloom, especially the spring bloomers, as too much nitrogen can affect the number of blooms. In veg garden, you'd want to use them on anything grass-like like corn but not anything that fruits (like tomatoes or peppers). For fruits like blueberries, wait until after they bloom and start bearing fruit. Should be good for compost at anytime as long as there's enough brown stuff in the pile.
