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 $235.47   8.709%   $255.9771   $255.98   $256.00   $256.00  
  Purchase #2 $13.58   5.620%   $14.3432   $14.34   $14.35   $14.30  
  Purchase #3 $86.81   7.422%   $93.253   $93.25   $93.25   $93.30  
  Purchase #4 $33.40   4.226%   $34.8115   $34.81   $34.80   $34.80  
  Purchase #5 $147.33   4.139%   $153.428   $153.43   $153.45   $153.40  
  Purchase #6 $111.71   4.698%   $116.9581   $116.96   $116.95   $117.00  
  Purchase #7 $55.88   8.711%   $60.7477   $60.75   $60.75   $60.70  
  7 purchase totals = $684.18       $729.5186   $729.52   $729.55   $729.50