Wanted to shiny hunt before the first gym so just wondering. Also, if there isn't any double grass, should I beat the gym leader first to increase the odds with double grass instead of regular random encounters to get a shiny? (what would you do?)
There is no double patches available before the first gym. Your chances of finding a shiny are never increased by the amount of badges you have too.
Source: I have shiny hunted myself, and I have played Black 2.