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 $123.39   3.795%   $128.0727   $128.07   $128.05   $128.10  
  Purchase #2 $11.55   12.210%   $12.9603   $12.96   $12.95   $13.00  
  Purchase #3 $16.73   12.254%   $18.7801   $18.78   $18.80   $18.80  
  Purchase #4 $166.29   3.164%   $171.5514   $171.55   $171.55   $171.60  
  Purchase #5 $231.31   11.854%   $258.7295   $258.73   $258.75   $258.70  
  Purchase #6 $28.98   4.169%   $30.1882   $30.19   $30.20   $30.20  
  Purchase #7 $27.68   5.536%   $29.2124   $29.21   $29.20   $29.20  
  7 purchase totals = $605.93       $649.4946   $649.49   $649.50   $649.60