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 $20.38   4.173%   $21.2305   $21.23   $21.25   $21.20  
  Purchase #2 $146.86   12.620%   $165.3937   $165.39   $165.40   $165.40  
  Purchase #3 $7.27   9.264%   $7.9435   $7.94   $7.95   $7.90  
  Purchase #4 $72.60   8.793%   $78.9837   $78.98   $79.00   $79.00  
  Purchase #5 $140.93   6.416%   $149.9721   $149.97   $149.95   $150.00  
  Purchase #6 $71.48   6.291%   $75.9768   $75.98   $76.00   $76.00  
  Purchase #7 $20.79   5.826%   $22.0012   $22.00   $22.00   $22.00  
  7 purchase totals = $480.31       $521.5015   $521.49   $521.55   $521.50