You should begin trading when you're getting valid trade signals and obvious when you're able to trade. Thus, if your backtesting results reveals you're more profitable starting your trading day @ 0930am est versus starting your trading day @ 1000am est...doesn't make sense to start your day after 1000am est.
Yet, don't let your backtest results be the only determining factor. Also, look at your real-trading results. Thus, if your backtest results say 0930am est is the best time to start your day but you have not been following your trading plan when you start @ 0930am est or you make too many trading mistakes...you need to stay on the sidelines and start your trading day @ 1000am est until you can fix the real-trading problems @ 0930am est.
My point, you're question is really only answer by YOUR trading plan and not answered by someone else that's not using the same trading plan as you.