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 $20.77   3.504%   $21.4978   $21.50   $21.50   $21.50  
  Purchase #2 $41.88   5.419%   $44.1495   $44.15   $44.15   $44.10  
  Purchase #3 $5.62   9.548%   $6.1566   $6.16   $6.15   $6.20  
  Purchase #4 $81.38   7.107%   $87.1637   $87.16   $87.15   $87.20  
  Purchase #5 $154.54   4.134%   $160.9287   $160.93   $160.95   $160.90  
  Purchase #6 $22.65   7.795%   $24.4156   $24.42   $24.40   $24.40  
  Purchase #7 $85.87   13.550%   $97.5054   $97.51   $97.50   $97.50  
  7 purchase totals = $412.71       $441.8173   $441.83   $441.80   $441.80