Yes, I was taught to do it staggered, but I think this was dying out at the time and I believe that by the time I wrote any letters of my own I didn't do the staggering. My theory is it's because of the prevalence of printed labels. I haven't seen it for a long time now.

Now that I'm reminiscing a bit, it was also fairly common at the time for people to order a batch of sender labels that they could affix to the envelope. My grandparents had particularly distinctive golden metallic labels which meant you could instantly tell who it was from (if you didn't already recognise the handwriting).