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 $133.26   3.306%   $137.6656   $137.67   $137.65   $137.70  
  Purchase #2 $33.30   13.342%   $37.7429   $37.74   $37.75   $37.70  
  Purchase #3 $24.74   6.382%   $26.3189   $26.32   $26.30   $26.30  
  Purchase #4 $233.49   8.137%   $252.4891   $252.49   $252.50   $252.50  
  Purchase #5 $18.45   4.228%   $19.2301   $19.23   $19.25   $19.20  
  Purchase #6 $24.78   12.451%   $27.8654   $27.87   $27.85   $27.90  
  Purchase #7 $136.88   3.497%   $141.6667   $141.67   $141.65   $141.70  
  7 purchase totals = $604.90       $642.9787   $642.99   $642.95   $643.00