I started using Agave "nectar" earlier this year when I got serious about avoiding artificial sweeteners, refined white sugar, and high fructose corn syrup (HFCS). Once we had our own honey supply, I quit using Agave. Shortly after I did this, I read two articles that explained how Agave is a fraud as far as natural sweeteners go; it is not a true nectar, and it does not necessarily have a low-glycemic index. Read more here.
DON'T FEED THE BEES
This article in Nourishing Traditions explains why you should avoid HFCS and agave. Furthermore, it says that honey "whether USDA-certified or not, in various grades, is only produced in the mid spring to late summer, when the biological materials are readily available for bees to digest, convert and regurgitate."
Honey production the rest of the year comes from "feeding bees refined sucrose or refined crystallized or liquid HFCS during the months when pollen and nectar are not available. The bees then run the sweetener through their digestive tracts. The resulting product is labeled as honey but it may not have the same quality as natural honey. Higher levels of refined fructose in honey cause accelerated Maillard browning reactions when heated above 140 degrees F."
I don't see how honey produced from feeding bees sugar could help with allergies. That comes from them feeding on pollen. So it does seem to me that sugar-fed bees produce a different product from those that gather nectar and pollen from plants.
After three days eating our honey, Emma, who measures her sinus congestion by how far up the piano keyboard she can sing, added three notes to her range. So we will add hives until we are able to harvest enough honey to meet our yearly needs. Until then, we will supplement by purchasing raw honey from local beekeepers who do not feed their bees.