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 $234.76   9.248%   $256.4706   $256.47   $256.45   $256.50  
  Purchase #2 $149.31   5.265%   $157.1712   $157.17   $157.15   $157.20  
  Purchase #3 $54.58   13.358%   $61.8708   $61.87   $61.85   $61.90  
  Purchase #4 $33.70   9.341%   $36.8479   $36.85   $36.85   $36.80  
  Purchase #5 $33.86   7.556%   $36.4185   $36.42   $36.40   $36.40  
  Purchase #6 $139.63   13.644%   $158.6811   $158.68   $158.70   $158.70  
  Purchase #7 $232.99   13.705%   $264.9213   $264.92   $264.90   $264.90  
  7 purchase totals = $878.83       $972.3814   $972.38   $972.30   $972.40