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 $250.98   6.368%   $266.9624   $266.96   $266.95   $267.00  
  Purchase #2 $224.58   3.865%   $233.26   $233.26   $233.25   $233.30  
  Purchase #3 $124.38   9.597%   $136.3167   $136.32   $136.30   $136.30  
  Purchase #4 $117.48   3.393%   $121.4661   $121.47   $121.45   $121.50  
  Purchase #5 $191.82   13.263%   $217.2611   $217.26   $217.25   $217.30  
  Purchase #6 $246.94   5.840%   $261.3613   $261.36   $261.35   $261.40  
  Purchase #7 $62.63   7.241%   $67.165   $67.17   $67.15   $67.20  
  7 purchase totals = $1,218.81       $1,303.7926   $1,303.80   $1,303.70   $1,304.00