In my testing I've found out that trades w bad alphas get stale really fast while stocks that have good IV's which result in higher aplhas has a little bit more shelf life. Case in point Nov 11 2011 weekly - AAPL and MA had really bad alphas, both of them sat for about 2 hours, huge pnl down on straddle. Contrast w another stock with an alpha of 1 has a little bit more staying power before it starts to head south/Buy trader an extra 1 hour to wait for move.