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 $131.67   11.462%   $146.762   $146.76   $146.75   $146.80  
  Purchase #2 $66.77   13.637%   $75.8754   $75.88   $75.90   $75.90  
  Purchase #3 $34.62   11.802%   $38.7059   $38.71   $38.70   $38.70  
  Purchase #4 $74.39   6.707%   $79.3793   $79.38   $79.40   $79.40  
  Purchase #5 $76.55   12.669%   $86.2481   $86.25   $86.25   $86.20  
  Purchase #6 $124.97   10.205%   $137.7232   $137.72   $137.70   $137.70  
  Purchase #7 $237.86   7.846%   $256.5225   $256.52   $256.50   $256.50  
  7 purchase totals = $746.83       $821.2164   $821.22   $821.20   $821.20