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 $19.66   5.150%   $20.6725   $20.67   $20.65   $20.70  
  Purchase #2 $187.25   6.307%   $199.0599   $199.06   $199.05   $199.10  
  Purchase #3 $19.52   10.135%   $21.4984   $21.50   $21.50   $21.50  
  Purchase #4 $109.29   3.474%   $113.0867   $113.09   $113.10   $113.10  
  Purchase #5 $29.73   7.671%   $32.0106   $32.01   $32.00   $32.00  
  Purchase #6 $129.83   9.389%   $142.0197   $142.02   $142.00   $142.00  
  Purchase #7 $143.90   9.520%   $157.5993   $157.60   $157.60   $157.60  
  7 purchase totals = $639.18       $685.9471   $685.95   $685.90   $686.00