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 $78.33   4.264%   $81.67   $81.67   $81.65   $81.70  
  Purchase #2 $32.80   10.461%   $36.2312   $36.23   $36.25   $36.20  
  Purchase #3 $70.97   12.545%   $79.8732   $79.87   $79.85   $79.90  
  Purchase #4 $136.49   4.395%   $142.4887   $142.49   $142.50   $142.50  
  Purchase #5 $202.28   7.517%   $217.4854   $217.49   $217.50   $217.50  
  Purchase #6 $229.26   5.433%   $241.7157   $241.72   $241.70   $241.70  
  Purchase #7 $216.78   11.136%   $240.9206   $240.92   $240.90   $240.90  
  7 purchase totals = $966.91       $1,040.3848   $1,040.39   $1,040.35   $1,040.40