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 $131.35   6.133%   $139.4057   $139.41   $139.40   $139.40  
  Purchase #2 $112.90   5.735%   $119.3748   $119.37   $119.35   $119.40  
  Purchase #3 $8.85   5.356%   $9.324   $9.32   $9.30   $9.30  
  Purchase #4 $6.51   11.594%   $7.2648   $7.26   $7.25   $7.30  
  Purchase #5 $130.28   4.228%   $135.7882   $135.79   $135.80   $135.80  
  Purchase #6 $107.65   5.205%   $113.2532   $113.25   $113.25   $113.30  
  Purchase #7 $27.91   8.850%   $30.38   $30.38   $30.40   $30.40  
  7 purchase totals = $525.45       $554.7907   $554.78   $554.75   $554.90