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Should I use regular or adjusted close for backtesting?
If you want to monitor how well you did in choosing your investments you will want to use stock prices that account for the dividends and splits and other changes (not just the closing price).  The adjusted close will include these changes where the straight close will not include them.  Using the adjusted close you will get your true percentage change.  For example I have a stock called PETS that paid an $0.18 dividend in July 2015. The adjusted closes before that day in July are all $0.18 lower per share.  Say the closing price had been unchanged at $20.00.  The close prices would say I made no profit, but the adjusted closing price would say I made $0.18 per share on this investment because the adjusted close would read $19.82 in June 2015 but would read $20.00 in August 2015 (just like the closing price).  The adjusted close allows me to know my true profit per share.