The observation regarding selling in arvo before ex dividend date can be attributed to foreign selling as they don't get the benefit of the franking credit unless they sell on the cum div market eg CBACD. My CFD account does something called a franking credit adjustment, I have never gone through it and the reading on the ASX is not very helpful either but my understanding it has a neutral effect.
Further stripping franking credits is a NO NO according to the ATO.
In trending markets it is ok to go either way like you suggest and I am thinking when NAB goes ex div I might go short too, but thats still a while to go.
