How do different spices react to heat?
I can't speak to the chemistry but I can describe the approaches I use in different spices and see if that helps. As Willem mentions in his comment, different cuisines may treat spices differently to bring out different aspects of the flavor.


Dried Herbs: typically I will add once I've begun the "watery/saucy" stage of a dish. I want these to simmer in the dish typically. Examples of these are Dried Oregano, Dried Rosemary, Dried Basil, Bay Leaves.
Fresh Herbs: typically I add these later in the cycle; many recipes will call for X tsp dried or X tbsp fresh of a particular herb. In these situations, my assumption why I prefer adding them later to the dish is that they are directly providing the flavor instead of diffusing the dried form into the sauce. I add cilantro as a finishing in both Mexican and Indian dishes. Similarly Basil is often added as a finishing ingredient.
Garlic: is a good example of multiple preparations used to bring out different aspects of the Garlic flavor. Roasting the garlic tends to mellow out the sharpness/spiciness of the garlic. This is done for some soups and often when making garlic butter for garlic bread. Raw garlic has a sharp kick that is used to flavor pestos. Sauteing garlic with onions or red peppers is a common starting point that isn't as sharp as raw garlic and not as creamy as the baked garlic.