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 $13.31   12.793%   $15.0127   $15.01   $15.00   $15.00  
  Purchase #2 $146.94   13.414%   $166.6505   $166.65   $166.65   $166.70  
  Purchase #3 $206.71   12.705%   $232.9725   $232.97   $232.95   $233.00  
  Purchase #4 $224.46   5.364%   $236.5   $236.50   $236.50   $236.50  
  Purchase #5 $19.60   8.614%   $21.2883   $21.29   $21.30   $21.30  
  Purchase #6 $68.94   7.428%   $74.0609   $74.06   $74.05   $74.10  
  Purchase #7 $202.91   3.610%   $210.2351   $210.24   $210.25   $210.20  
  7 purchase totals = $882.87       $956.7200   $956.72   $956.70   $956.80