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 $17.91   9.273%   $19.5708   $19.57   $19.55   $19.60  
  Purchase #2 $125.53   11.118%   $139.4864   $139.49   $139.50   $139.50  
  Purchase #3 $130.86   9.714%   $143.5717   $143.57   $143.55   $143.60  
  Purchase #4 $192.92   8.768%   $209.8352   $209.84   $209.85   $209.80  
  Purchase #5 $173.45   8.592%   $188.3528   $188.35   $188.35   $188.40  
  Purchase #6 $9.71   10.758%   $10.7546   $10.75   $10.75   $10.80  
  Purchase #7 $159.71   12.208%   $179.2074   $179.21   $179.20   $179.20  
  7 purchase totals = $810.09       $890.7789   $890.78   $890.75   $890.90