Everytime I install a new Rom, I double check to make sure it has a boot.img, then upon using CWM to install it, I first do the following(some of them are redundant, but I do so for good measure):
- wipe data/factory reset
- wipe cache partition
- advanced > Wipe Dalvik Cache
- advanced > Wipe Battery Stats
- mounts and storage > unmount cache
- mounts and storage > format /boot (If the rom had a boot.img)
- mounts and storage > format /cache
- mounts and storage > format /data
- mounts and storage > format /system
Then I install the ROM zip, and if I want GAPPS, I flash those afterward.
This ensures that ALL data is gone, and it will not allow for unique bugs caused by remaining data from a previous rom.