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.