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 $18.61   11.629%   $20.7742   $20.77   $20.75   $20.80  
  Purchase #2 $12.88   4.227%   $13.4244   $13.42   $13.40   $13.40  
  Purchase #3 $107.71   4.463%   $112.5171   $112.52   $112.50   $112.50  
  Purchase #4 $165.38   8.650%   $179.6854   $179.69   $179.70   $179.70  
  Purchase #5 $121.76   12.743%   $137.2759   $137.28   $137.30   $137.30  
  Purchase #6 $113.39   11.558%   $126.4956   $126.50   $126.50   $126.50  
  Purchase #7 $113.84   10.382%   $125.6589   $125.66   $125.65   $125.70  
  7 purchase totals = $653.57       $715.8315   $715.84   $715.80   $715.90