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 $125.63   9.834%   $137.9845   $137.98   $138.00   $138.00  
  Purchase #2 $135.93   11.796%   $151.9643   $151.96   $151.95   $152.00  
  Purchase #3 $241.30   5.154%   $253.7366   $253.74   $253.75   $253.70  
  Purchase #4 $119.51   5.266%   $125.8034   $125.80   $125.80   $125.80  
  Purchase #5 $15.78   12.603%   $17.7688   $17.77   $17.75   $17.80  
  Purchase #6 $27.83   9.308%   $30.4204   $30.42   $30.40   $30.40  
  Purchase #7 $16.26   12.833%   $18.3466   $18.35   $18.35   $18.30  
  7 purchase totals = $682.24       $736.0246   $736.02   $736.00   $736.00