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 $213.44   12.332%   $239.7614   $239.76   $239.75   $239.80  
  Purchase #2 $44.46   7.265%   $47.69   $47.69   $47.70   $47.70  
  Purchase #3 $50.77   10.105%   $55.9003   $55.90   $55.90   $55.90  
  Purchase #4 $195.63   11.669%   $218.4581   $218.46   $218.45   $218.50  
  Purchase #5 $207.64   11.280%   $231.0618   $231.06   $231.05   $231.10  
  Purchase #6 $14.62   5.172%   $15.3761   $15.38   $15.40   $15.40  
  Purchase #7 $64.57   12.467%   $72.6199   $72.62   $72.60   $72.60  
  7 purchase totals = $791.13       $880.8676   $880.87   $880.85   $881.00