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 $17.56   3.528%   $18.1795   $18.18   $18.20   $18.20  
  Purchase #2 $18.99   10.393%   $20.9636   $20.96   $20.95   $21.00  
  Purchase #3 $109.40   7.821%   $117.9562   $117.96   $117.95   $118.00  
  Purchase #4 $146.61   7.268%   $157.2656   $157.27   $157.25   $157.30  
  Purchase #5 $136.82   10.558%   $151.2655   $151.27   $151.25   $151.30  
  Purchase #6 $110.72   5.824%   $117.1683   $117.17   $117.15   $117.20  
  Purchase #7 $118.94   8.540%   $129.0975   $129.10   $129.10   $129.10  
  7 purchase totals = $659.04       $711.8962   $711.91   $711.85   $712.10