Like in hops, flavor additions should be near flame-out, so that you get some 'steep time' for the flavors to transfer into the wort, like a tea. Boiling too long will break down some of the more delicate aromas & flavors. The heavy particles will settle out to the trub and not be in your final product.
If you have any spices or herbs that contribute bitterness, then those should probably be boiled a bit to extract the bitterness well.
Also note that spice flavors may be 'sharp' or 'harsh' when the beer is young, but mellow out significantly over several months of aging.