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 $7.98   12.845%   $9.005   $9.01   $9.00   $9.00  
  Purchase #2 $9.79   9.426%   $10.7128   $10.71   $10.70   $10.70  
  Purchase #3 $34.93   11.329%   $38.8872   $38.89   $38.90   $38.90  
  Purchase #4 $200.60   11.263%   $223.1936   $223.19   $223.20   $223.20  
  Purchase #5 $74.88   4.185%   $78.0137   $78.01   $78.00   $78.00  
  Purchase #6 $117.58   3.560%   $121.7658   $121.77   $121.75   $121.80  
  Purchase #7 $183.28   7.730%   $197.4475   $197.45   $197.45   $197.40  
  7 purchase totals = $629.04       $679.0256   $679.03   $679.00   $679.00