Hi Lynne - first of all, HUGE Congratulations at getting them all.
Now, how rare is a ribbed back? On a stippled Strawberry, funnily enough that is the back pattern that is mostly seen. On the unstippled pieces there are possibly more basketweave backs. Just to mis-prove that, my stippled Strawberry bowl has a basketweave back!
It's more a matter of curiosity really - I don't think it would really make a big difference to the value. What is far more important is that it is stippled. The stippled pieces (same in other patterns where there are unstippled and stippled variants) are always more sought after because - well - they look better. Stippling refracts the light more and makes the iridescence showier.
A great buy. Well done and my congratulations again.