I'd say do it all at once in one go as you won't lose out on the wish credits. Once the rebate is over and you only did it one level at a time and wait to get more runes and level again, you would lose potential wish credits which would potentially benefit you in the future, which is the reason u should do it all in one go. Usually I go from lvl 7-10 or you can even go lvl 6-10