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 $226.47   5.844%   $239.7049   $239.70   $239.70   $239.70  
  Purchase #2 $143.88   7.527%   $154.7098   $154.71   $154.70   $154.70  
  Purchase #3 $63.50   11.374%   $70.7225   $70.72   $70.70   $70.70  
  Purchase #4 $113.47   5.209%   $119.3807   $119.38   $119.40   $119.40  
  Purchase #5 $34.34   6.117%   $36.4406   $36.44   $36.45   $36.40  
  Purchase #6 $183.90   11.735%   $205.4807   $205.48   $205.50   $205.50  
  Purchase #7 $6.40   10.135%   $7.0486   $7.05   $7.05   $7.00  
  7 purchase totals = $771.96       $833.4878   $833.48   $833.50   $833.40