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 $8.40   12.106%   $9.4169   $9.42   $9.40   $9.40  
  Purchase #2 $37.41   5.603%   $39.5061   $39.51   $39.50   $39.50  
  Purchase #3 $46.47   3.459%   $48.0774   $48.08   $48.10   $48.10  
  Purchase #4 $19.59   9.854%   $21.5204   $21.52   $21.50   $21.50  
  Purchase #5 $51.47   5.606%   $54.3554   $54.36   $54.35   $54.40  
  Purchase #6 $222.88   7.437%   $239.4556   $239.46   $239.45   $239.50  
  Purchase #7 $164.76   8.526%   $178.8074   $178.81   $178.80   $178.80  
  7 purchase totals = $550.98       $591.1392   $591.16   $591.10   $591.20