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 $64.71   5.667%   $68.3771   $68.38   $68.40   $68.40  
  Purchase #2 $215.91   12.144%   $242.1301   $242.13   $242.15   $242.10  
  Purchase #3 $41.49   13.770%   $47.2032   $47.20   $47.20   $47.20  
  Purchase #4 $29.61   4.412%   $30.9164   $30.92   $30.90   $30.90  
  Purchase #5 $77.83   12.649%   $87.6747   $87.67   $87.65   $87.70  
  Purchase #6 $204.74   3.801%   $212.5222   $212.52   $212.50   $212.50  
  Purchase #7 $106.37   9.435%   $116.406   $116.41   $116.40   $116.40  
  7 purchase totals = $740.66       $805.2297   $805.23   $805.20   $805.20