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 $137.56   12.393%   $154.6078   $154.61   $154.60   $154.60  
  Purchase #2 $55.38   5.206%   $58.2631   $58.26   $58.25   $58.30  
  Purchase #3 $118.49   9.723%   $130.0108   $130.01   $130.00   $130.00  
  Purchase #4 $109.63   8.207%   $118.6273   $118.63   $118.65   $118.60  
  Purchase #5 $161.66   3.864%   $167.9065   $167.91   $167.90   $167.90  
  Purchase #6 $65.37   11.819%   $73.0961   $73.10   $73.10   $73.10  
  Purchase #7 $162.48   3.694%   $168.482   $168.48   $168.50   $168.50  
  7 purchase totals = $810.57       $870.9936   $871.00   $871.00   $871.00