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 $18.64   9.411%   $20.3942   $20.39   $20.40   $20.40  
  Purchase #2 $21.64   10.456%   $23.9027   $23.90   $23.90   $23.90  
  Purchase #3 $44.36   12.364%   $49.8447   $49.84   $49.85   $49.80  
  Purchase #4 $130.71   4.658%   $136.7985   $136.80   $136.80   $136.80  
  Purchase #5 $131.34   5.212%   $138.1854   $138.19   $138.20   $138.20  
  Purchase #6 $117.54   3.123%   $121.2108   $121.21   $121.20   $121.20  
  Purchase #7 $136.68   10.790%   $151.4278   $151.43   $151.45   $151.40  
  7 purchase totals = $600.91       $641.7641   $641.76   $641.80   $641.70