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 $5.65   9.800%   $6.2037   $6.20   $6.20   $6.20  
  Purchase #2 $154.40   10.152%   $170.0747   $170.07   $170.05   $170.10  
  Purchase #3 $190.92   9.353%   $208.7767   $208.78   $208.80   $208.80  
  Purchase #4 $113.59   7.351%   $121.94   $121.94   $121.95   $121.90  
  Purchase #5 $141.74   10.103%   $156.06   $156.06   $156.05   $156.10  
  Purchase #6 $14.89   5.138%   $15.655   $15.66   $15.65   $15.70  
  Purchase #7 $132.27   12.186%   $148.3884   $148.39   $148.40   $148.40  
  7 purchase totals = $753.46       $827.0985   $827.10   $827.10   $827.20