There's no specific strategy that's been defined as optimal in these tournaments, to my knowledge. However, I've seen strong evidence from large samples of multiple players playing rebuy SNGs which seems to indicate that the best strategy in terms of ROI is to simply get to the add-on stage of the tournament as cheaply as possible. It's unclear if this is also best in terms of hourly rate, and things are going to change pretty drastically when Stars starts raking rebuys for many countries in 2015, but for the time being, that's the idea.

When it comes to what stack depth you should rebuy, I'm honestly not sure. I tend to just click double auto-rebuy and get to the add-on stage no matter what, because the 1r 2x/3r 3x/8r 2x are all pretty soft tournaments and I feel like I'm throwing away an edge if I spend $X on entering a soft tournament and deliberately leave it before the add-on. There are so many weak players who do exactly that, that the dead money in the prize pool makes them pretty good value. However, there's a decent likelihood that what I'm doing is suboptimal, so I'm definitely willing to listen to other theories.