Every fiber of my being believes it is proper to spread the peanut butter side of the sandwich first. However, getting peanut butter off a knife is hard work. Cleaning jelly off a knife is quick and easy, so obviously one *should* spread the jelly first. Then safely proceed to the peanut butter, free of worry that cross-contamination might occur.
But that would be a jelly peanut butter sandwich, and that's just wrong.