Can I use unbleached flour in quickbread?
EDIT

Cook's Illustrated (America's Test Kitchen) tested bleached versus unbleached flour in baking and in savory sauces.

They found that unbleached flour works pretty much identically to bleached flour in sauces, but they caution that some testers could taste off notes in some baked applications using bleached flour. The difference is more pronounced in applications where the ratio of flour to other ingredients is high.


  In baking tests, bleached flour was criticized for tasting flat or having “off” flavors (texturally, the flours behaved the same). These characteristics, however, were much harder to detect in recipes with a high proportion of ingredients other than flour, such as cornbread or oatmeal cookies.


According to ATK, unbleached flour is preferable to bleached. You may find that you actually like your bread better using unbleached flour.

Interestingly, the link in @dougal 2.0.0's answer directly contradicts ATK. Kitchn prefers bleached flour for quickbreads, claiming that bleached flour is softer.

So, knowledgeable cooks disagree. I will say that I frequently make quick breads including zucchini. I always use unbleached flour, and I've never seen any reason to buy bleached.  

Original Answer

To the absolute best of my knowledge, I don't think there is a difference between unbleached and bleached flours in a particular application other than what quickly becomes obvious.

So, if you have used both flours in your bread, I see no reason other than preference or economy to use one or the other.