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 $60.99   8.836%   $66.3791   $66.38   $66.40   $66.40  
  Purchase #2 $102.46   9.837%   $112.539   $112.54   $112.55   $112.50  
  Purchase #3 $108.73   9.454%   $119.0093   $119.01   $119.00   $119.00  
  Purchase #4 $117.73   13.418%   $133.527   $133.53   $133.55   $133.50  
  Purchase #5 $190.80   11.713%   $213.1484   $213.15   $213.15   $213.10  
  Purchase #6 $18.52   11.815%   $20.7081   $20.71   $20.70   $20.70  
  Purchase #7 $15.60   4.266%   $16.2655   $16.27   $16.25   $16.30  
  7 purchase totals = $614.83       $681.5764   $681.59   $681.60   $681.50