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 $114.92   12.616%   $129.4183   $129.42   $129.40   $129.40  
  Purchase #2 $114.88   13.758%   $130.6852   $130.69   $130.70   $130.70  
  Purchase #3 $78.90   9.542%   $86.4286   $86.43   $86.45   $86.40  
  Purchase #4 $141.33   3.300%   $145.9939   $145.99   $146.00   $146.00  
  Purchase #5 $164.55   3.127%   $169.6955   $169.70   $169.70   $169.70  
  Purchase #6 $20.26   8.133%   $21.9077   $21.91   $21.90   $21.90  
  Purchase #7 $138.69   12.654%   $156.2398   $156.24   $156.25   $156.20  
  7 purchase totals = $773.53       $840.3690   $840.38   $840.40   $840.30