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 $52.64   13.527%   $59.7606   $59.76   $59.75   $59.80  
  Purchase #2 $31.36   12.207%   $35.1881   $35.19   $35.20   $35.20  
  Purchase #3 $113.83   7.366%   $122.2147   $122.21   $122.20   $122.20  
  Purchase #4 $135.49   11.669%   $151.3003   $151.30   $151.30   $151.30  
  Purchase #5 $83.59   13.721%   $95.0594   $95.06   $95.05   $95.10  
  Purchase #6 $116.73   7.347%   $125.3062   $125.31   $125.30   $125.30  
  Purchase #7 $50.82   11.149%   $56.4859   $56.49   $56.50   $56.50  
  7 purchase totals = $584.46       $645.3152   $645.32   $645.30   $645.40