It all depends on the chemical constituents you want. The anti-flu properties of elderberry, for example, could care less about heat. The anit-inflammatory properties of aucubin (found in many plants) is destroyed by heat.
I teach to errr on the side of caution. Warming an infused oil in in a window sill, in a brown paper bag is fine for most things. Doing a tea where you throw the herbs in boiling water and then pull it off the heat immediately is fine for most herbs. Simmering herbs for a long time (decoction) is fine for many herbs and completely destructive for others.
In the old days, they used to do a lot of decoction. Decoction is used for tough, woody material like roots or bark. These days, when we have nice blenders that can powder anything, there is no reason to do decoctions…just do teas.
Don't use herbs or combine herbs with medications or use them during lactation or pregnancy without talking with your healthcare provider.