I think it depends on the bow and how it is shooting,,
the string may be down the center,, but one of the limbs as top limb will probably shoot better or be easier to tune, or shoot a wider range of spine,,
I have found that it may "look". one way,, as far as string alignment ,,,but shooting the bow will tell you otherwise,,
so that would be my main determining factor and I would adjust the tiller if needed,,, putting the stiff limb where ever the bow shot best,,
if that was on top I would probably adjust the tiller if needed to get a bit of positive tiller,,
I usually put a limb that has a bit more reflex on the top,, in my experience,, after a lot of hard shooting, ,the limb with the most reflex is most likely to give in a bit or shift a bit,,, if it is on top,, and the tiller becomes a bit more positive,,then little or no adjustment may be needed to shoot well,,,,, if that limb is on the bottom,, and gives in a bit,, you have lost your positive tiller,, and it could effect the way the bow is shooting,, a longer bow is not as sensitive,,, but if the bow is a bit short,, the tiller may have to be adjusted to get perfect arrow flight,,,
that being said,, good points above,, that is just my preference based on my experience,,and have not doubt there are many ways to skin cat in this situation