Add spices/herbs early or late to a chili or stew in a crockpot slow-cooker?
As a general rule they should go in early for a couple of reasons:

Flavours don't really cook out, they get distributed within the dish. So adding spices etc. early allows the flavours to combine. If they did cook out you could just add a touch more. 

Opening a slow cooker during the cooking time lets write a lot of heat out, at best extending the cooking time. Repeatedly opening it from early in the cook (worst case) would bring the temperature down too far. So you either add things at the beginning or at the end (when the food is already cooked and hot enough all the way through that it will stay at a sensible temperature). This allows final fine adjustment at the end, but you may need to be choosy about the form in which you add the heat - some things could be hard or gritty if they haven't cooked. Fresh herbs can be added just before serving as they don't need to cook and may lose some of the fresh components of their flavour.