I am not an expert on SRing. I haven't had a successful shiny by that method as i find it very annoying. I am going to try to SR in black 2 for a shiny Latios though because i have the shiny charm so odds are better.
However it does not matter about switching the DS off by the power button at all because the game seed will be reset whichever way you do it. Like has been said it is all to do with timing.