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 $89.68   12.465%   $100.8586   $100.86   $100.85   $100.90  
  Purchase #2 $90.89   7.510%   $97.7158   $97.72   $97.70   $97.70  
  Purchase #3 $141.37   5.870%   $149.6684   $149.67   $149.65   $149.70  
  Purchase #4 $76.42   7.159%   $81.8909   $81.89   $81.90   $81.90  
  Purchase #5 $141.27   3.266%   $145.8839   $145.88   $145.90   $145.90  
  Purchase #6 $68.54   7.783%   $73.8745   $73.87   $73.85   $73.90  
  Purchase #7 $28.87   4.121%   $30.0597   $30.06   $30.05   $30.10  
  7 purchase totals = $637.04       $679.9518   $679.95   $679.90   $680.10