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 $68.42   6.351%   $72.7654   $72.77   $72.75   $72.80  
  Purchase #2 $224.93   8.858%   $244.8543   $244.85   $244.85   $244.90  
  Purchase #3 $230.63   13.583%   $261.9565   $261.96   $261.95   $262.00  
  Purchase #4 $23.96   9.481%   $26.2316   $26.23   $26.25   $26.20  
  Purchase #5 $14.89   11.646%   $16.6241   $16.62   $16.60   $16.60  
  Purchase #6 $108.40   7.301%   $116.3143   $116.31   $116.30   $116.30  
  Purchase #7 $236.50   7.273%   $253.7006   $253.70   $253.70   $253.70  
  7 purchase totals = $907.73       $992.4468   $992.44   $992.40   $992.50