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 $18.60   11.827%   $20.7998   $20.80   $20.80   $20.80  
  Purchase #2 $187.52   5.863%   $198.5143   $198.51   $198.50   $198.50  
  Purchase #3 $39.51   9.626%   $43.3132   $43.31   $43.30   $43.30  
  Purchase #4 $184.89   3.393%   $191.1633   $191.16   $191.15   $191.20  
  Purchase #5 $12.50   13.772%   $14.2215   $14.22   $14.20   $14.20  
  Purchase #6 $140.90   3.601%   $145.9738   $145.97   $145.95   $146.00  
  Purchase #7 $68.91   10.662%   $76.2572   $76.26   $76.25   $76.30  
  7 purchase totals = $652.83       $690.2431   $690.23   $690.15   $690.30