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 $50.51   12.323%   $56.7343   $56.73   $56.75   $56.70  
  Purchase #2 $232.77   7.813%   $250.9563   $250.96   $250.95   $251.00  
  Purchase #3 $93.57   8.208%   $101.2502   $101.25   $101.25   $101.30  
  Purchase #4 $237.82   10.646%   $263.1383   $263.14   $263.15   $263.10  
  Purchase #5 $177.28   12.644%   $199.6953   $199.70   $199.70   $199.70  
  Purchase #6 $209.49   6.405%   $222.9078   $222.91   $222.90   $222.90  
  Purchase #7 $230.29   13.707%   $261.8559   $261.86   $261.85   $261.90  
  7 purchase totals = $1,231.73       $1,356.5381   $1,356.55   $1,356.55   $1,356.60