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 $55.83   12.228%   $62.6569   $62.66   $62.65   $62.70  
  Purchase #2 $29.57   7.586%   $31.8132   $31.81   $31.80   $31.80  
  Purchase #3 $74.36   11.807%   $83.1397   $83.14   $83.15   $83.10  
  Purchase #4 $29.76   9.483%   $32.5821   $32.58   $32.60   $32.60  
  Purchase #5 $229.74   13.677%   $261.1615   $261.16   $261.15   $261.20  
  Purchase #6 $186.47   4.797%   $195.415   $195.42   $195.40   $195.40  
  Purchase #7 $158.66   9.684%   $174.0246   $174.02   $174.00   $174.00  
  7 purchase totals = $764.39       $840.7930   $840.79   $840.75   $840.80