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 $47.48   3.475%   $49.1299   $49.13   $49.15   $49.10  
  Purchase #2 $19.92   5.552%   $21.026   $21.03   $21.05   $21.00  
  Purchase #3 $169.71   6.242%   $180.3033   $180.30   $180.30   $180.30  
  Purchase #4 $80.87   6.658%   $86.2543   $86.25   $86.25   $86.30  
  Purchase #5 $135.74   8.729%   $147.5887   $147.59   $147.60   $147.60  
  Purchase #6 $69.96   4.186%   $72.8885   $72.89   $72.90   $72.90  
  Purchase #7 $76.84   9.674%   $84.2735   $84.27   $84.25   $84.30  
  7 purchase totals = $600.52       $641.4642   $641.46   $641.50   $641.50