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 $248.36   8.576%   $269.6594   $269.66   $269.65   $269.70  
  Purchase #2 $14.39   9.381%   $15.7399   $15.74   $15.75   $15.70  
  Purchase #3 $240.67   9.437%   $263.382   $263.38   $263.40   $263.40  
  Purchase #4 $130.59   5.277%   $137.4812   $137.48   $137.50   $137.50  
  Purchase #5 $68.84   9.146%   $75.1361   $75.14   $75.15   $75.10  
  Purchase #6 $68.55   11.711%   $76.5779   $76.58   $76.60   $76.60  
  Purchase #7 $16.90   12.518%   $19.0155   $19.02   $19.00   $19.00  
  7 purchase totals = $788.30       $856.9920   $857.00   $857.05   $857.00