The only sweetener called for in the bread recipe I want to use is honey; can I substitute 1/3 a cup of agave nectar for 1/3 a cup of honey without throwing off the proportions of the recipe? When using agave nectar as a substitute for honey in salad dressing recipes, I tend to use less agave nectar than honey called for because I find agave nectar sweeter than honey, but I don't feel confident doing the same thing in baking.
You can use agave nectar instead of honey or sugar if you modify your recipe a bit by lowering the amount of liquids. Agave nectar has more water than honey and is sweeter than honey (about 50% sweeter).
Honey Fructose: 41%, Glucose: 36%, Water: 18%
Agave nectar Fructose: 53%, Glucose: 15%, Water: 25%
As a starting point in modifying the recipe, keep the amount of water fixed by using less agave nectar than honey (amount of agave = 0.72 amount of honey). Also, dishes made with agave nectar will brown faster than those made with honey because of the agave's higher fructose content. Start checking a little earlier.
If you're making some sort of quick bread or muffin recipe, you should be fine. I've never had any problem and I can't see why it'd make much difference other than in the sweetness. Agave nectar seems a little thinner than honey to me, so that probably means the ratio of sugars to water is lower; however, that doesn't mean it's necessarily less sweet, and as you say it seems a little sweeter to me (mostly because it lacks that back-of-the-throat twang that honey has).
If you're making something that requires extreme chemical precision, I suppose there might be a problem, but then honey varies all over the map in terms of density and taste anyway. (edit — I just noticed the word "bread" in your question - if it's a yeast bread, I would be stunned if that substitution would make any difference at all; it's such a small amount.)
I used agave in a yeast bread recipe in my bread machine. With the small amount of honey needed (1, 3/4 Tbsp) I could substitute an equal amount of agave with perfect results.