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 $129.74   4.379%   $135.4213   $135.42   $135.40   $135.40  
  Purchase #2 $133.40   8.214%   $144.3575   $144.36   $144.35   $144.40  
  Purchase #3 $7.35   12.131%   $8.2416   $8.24   $8.25   $8.20  
  Purchase #4 $20.47   12.817%   $23.0936   $23.09   $23.10   $23.10  
  Purchase #5 $143.54   7.168%   $153.8289   $153.83   $153.85   $153.80  
  Purchase #6 $11.71   13.636%   $13.3068   $13.31   $13.30   $13.30  
  Purchase #7 $69.47   9.636%   $76.1641   $76.16   $76.15   $76.20  
  7 purchase totals = $515.68       $554.4138   $554.41   $554.40   $554.40