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 $52.87   11.599%   $59.0024   $59.00   $59.00   $59.00  
  Purchase #2 $57.26   7.123%   $61.3386   $61.34   $61.35   $61.30  
  Purchase #3 $230.87   12.254%   $259.1608   $259.16   $259.15   $259.20  
  Purchase #4 $239.53   9.198%   $261.562   $261.56   $261.55   $261.60  
  Purchase #5 $76.65   4.423%   $80.0402   $80.04   $80.05   $80.00  
  Purchase #6 $109.56   3.129%   $112.9881   $112.99   $113.00   $113.00  
  Purchase #7 $117.46   5.662%   $124.1106   $124.11   $124.10   $124.10  
  7 purchase totals = $884.20       $958.2027   $958.20   $958.20   $958.20