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 $15.49   4.495%   $16.1863   $16.19   $16.20   $16.20  
  Purchase #2 $147.44   7.655%   $158.7265   $158.73   $158.75   $158.70  
  Purchase #3 $28.90   10.541%   $31.9463   $31.95   $31.95   $31.90  
  Purchase #4 $143.68   5.386%   $151.4186   $151.42   $151.40   $151.40  
  Purchase #5 $93.54   8.128%   $101.1429   $101.14   $101.15   $101.10  
  Purchase #6 $17.36   13.573%   $19.7163   $19.72   $19.70   $19.70  
  Purchase #7 $141.94   13.840%   $161.5845   $161.58   $161.60   $161.60  
  7 purchase totals = $588.35       $640.7214   $640.73   $640.75   $640.60