The Builds are No WIPE, but as a matter of fact, if we don't wipe there seems to be bad performance issues. I have upgraded to Build 8 without a wipe and it seems to terribly slow down my device. Same with Build 7 when I upgraded from Build 6. It was smooth as butter after wiping and installing Build 7.
Since this is still in Beta phase, it would be best to force people to wipe on each flash so that bugs are minimal.