my answers would be :
1 - Nightlies are not release canditates nor stable versions but it is advisable if you want to test them - if you want a full 100% stable version, i would say it's better to wait.
2 - I always wipe cache and dalvik between nightlies to avoid risks - but no data reset which is in my opinion usefull only if you change your base system (and not for updates), but i'm no dev, only user.
Thanx to the dev's for the work - as a simple user, i'm everyday (and every nightly) amazed by the work done
SGS2 with WanamLite v14.4 JB & Speedmod K5-8
G-tab p1000 pimped by CM (10.1)