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 $21.50   3.367%   $22.2239   $22.22   $22.20   $22.20  
  Purchase #2 $10.30   12.824%   $11.6209   $11.62   $11.60   $11.60  
  Purchase #3 $112.77   8.104%   $121.9089   $121.91   $121.90   $121.90  
  Purchase #4 $234.81   11.858%   $262.6538   $262.65   $262.65   $262.70  
  Purchase #5 $37.83   11.872%   $42.3212   $42.32   $42.30   $42.30  
  Purchase #6 $86.63   10.516%   $95.74   $95.74   $95.75   $95.70  
  Purchase #7 $77.97   9.486%   $85.3662   $85.37   $85.35   $85.40  
  7 purchase totals = $581.81       $641.8349   $641.83   $641.75   $641.80