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 $18.93   9.626%   $20.7522   $20.75   $20.75   $20.80  
  Purchase #2 $209.46   10.315%   $231.0658   $231.07   $231.05   $231.10  
  Purchase #3 $240.61   10.400%   $265.6334   $265.63   $265.65   $265.60  
  Purchase #4 $110.69   5.273%   $116.5267   $116.53   $116.55   $116.50  
  Purchase #5 $125.41   12.114%   $140.6022   $140.60   $140.60   $140.60  
  Purchase #6 $75.27   6.469%   $80.1392   $80.14   $80.15   $80.10  
  Purchase #7 $29.25   6.590%   $31.1776   $31.18   $31.20   $31.20  
  7 purchase totals = $809.62       $885.8971   $885.90   $885.95   $885.90