I just flashed the 09/15 nightly. This time, instead of using Rom Manager to boot into recovery and flash the ROM, I just resbooted into recovery manually and chose the nightly ROM. I did NO resets or wipes before this. After the ROM flashed, I immediately chose the GAPPS file and flashed those. Restarted, Android updated the ~180 apps, and it was ready to go. Didn't have sign back into my Google account, Play and Google Music work great.
Unless I run into problems, or there is some major file system change, I'm not going to do any wipes or resets before flashing the nightly updates.