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 $60.50   13.434%   $68.6276   $68.63   $68.65   $68.60  
  Purchase #2 $15.45   8.514%   $16.7654   $16.77   $16.75   $16.80  
  Purchase #3 $219.62   13.475%   $249.2138   $249.21   $249.20   $249.20  
  Purchase #4 $137.50   8.320%   $148.94   $148.94   $148.95   $148.90  
  Purchase #5 $247.97   12.479%   $278.9142   $278.91   $278.90   $278.90  
  Purchase #6 $162.88   3.416%   $168.444   $168.44   $168.45   $168.40  
  Purchase #7 $85.62   6.293%   $91.0081   $91.01   $91.00   $91.00  
  7 purchase totals = $929.54       $1,021.9131   $1,021.91   $1,021.90   $1,021.80