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 $18.76   11.306%   $20.881   $20.88   $20.90   $20.90  
  Purchase #2 $18.77   11.674%   $20.9612   $20.96   $20.95   $21.00  
  Purchase #3 $209.82   12.292%   $235.6111   $235.61   $235.60   $235.60  
  Purchase #4 $81.27   13.675%   $92.3837   $92.38   $92.40   $92.40  
  Purchase #5 $112.28   4.171%   $116.9632   $116.96   $116.95   $117.00  
  Purchase #6 $75.54   13.669%   $85.8656   $85.87   $85.85   $85.90  
  Purchase #7 $76.60   3.581%   $79.343   $79.34   $79.35   $79.30  
  7 purchase totals = $593.04       $652.0088   $652.00   $652.00   $652.10