does it really matter? we sent a mix printed on the front, we wrote on the back, some had nothing on them. I dont think they care as long as they can see when they were taken, or at least they can see from the photos as well that there was time between the photos being taken. ( you have aged for example).
Its not like their using then for your ID photos or anything, they just want to see that you were in a relationship when you say you were. ;D