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 $100.34   5.116%   $105.4734   $105.47   $105.45   $105.50  
  Purchase #2 $78.39   7.735%   $84.4535   $84.45   $84.45   $84.50  
  Purchase #3 $117.61   9.804%   $129.1405   $129.14   $129.15   $129.10  
  Purchase #4 $107.71   12.319%   $120.9788   $120.98   $121.00   $121.00  
  Purchase #5 $92.49   5.485%   $97.5631   $97.56   $97.55   $97.60  
  Purchase #6 $84.30   7.169%   $90.3435   $90.34   $90.35   $90.30  
  Purchase #7 $30.46   5.375%   $32.0972   $32.10   $32.10   $32.10  
  7 purchase totals = $611.30       $660.0500   $660.04   $660.05   $660.10