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 $20.54   8.785%   $22.3444   $22.34   $22.35   $22.30  
  Purchase #2 $139.39   12.772%   $157.1929   $157.19   $157.20   $157.20  
  Purchase #3 $70.98   8.743%   $77.1858   $77.19   $77.20   $77.20  
  Purchase #4 $18.96   3.552%   $19.6335   $19.63   $19.65   $19.60  
  Purchase #5 $112.27   8.583%   $121.9061   $121.91   $121.90   $121.90  
  Purchase #6 $124.47   5.777%   $131.6606   $131.66   $131.65   $131.70  
  Purchase #7 $82.31   3.581%   $85.2575   $85.26   $85.25   $85.30  
  7 purchase totals = $568.92       $615.1808   $615.18   $615.20   $615.20