Others may differ, but I've been making symmetrical bows for years, and I've never made one limb different than the other, I just don't see any difference down range either way. Especially if this is your first bow, don't overcomplicate it. If you really want to do this though, just shift the bow to one side on your tillering tree so that you are pulling on the string where your arrow will pass, then continue tillering as normal, making sure that the limbs are bending evenly, and that the tips end up at the same location on your tillering board grid lines.
Does that make sense?