Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $244.59   12.807%   $275.9146   $275.91   $275.90   $275.90  
  Purchase #2 $90.65   13.272%   $102.6811   $102.68   $102.70   $102.70  
  Purchase #3 $61.87   4.542%   $64.6801   $64.68   $64.70   $64.70  
  Purchase #4 $176.87   8.415%   $191.7536   $191.75   $191.75   $191.80  
  Purchase #5 $104.71   3.143%   $108.001   $108.00   $108.00   $108.00  
  Purchase #6 $234.94   11.491%   $261.937   $261.94   $261.95   $261.90  
  Purchase #7 $119.59   12.568%   $134.6201   $134.62   $134.60   $134.60  
  7 purchase totals = $1,033.22       $1,139.5875   $1,139.58   $1,139.60   $1,139.60