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 $247.66   4.586%   $259.0177   $259.02   $259.00   $259.00  
  Purchase #2 $122.61   13.669%   $139.3696   $139.37   $139.35   $139.40  
  Purchase #3 $51.31   12.543%   $57.7458   $57.75   $57.75   $57.70  
  Purchase #4 $69.75   5.610%   $73.663   $73.66   $73.65   $73.70  
  Purchase #5 $112.86   5.433%   $118.9917   $118.99   $119.00   $119.00  
  Purchase #6 $11.74   8.543%   $12.7429   $12.74   $12.75   $12.70  
  Purchase #7 $225.30   13.506%   $255.729   $255.73   $255.75   $255.70  
  7 purchase totals = $841.23       $917.2597   $917.26   $917.25   $917.20