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 $129.77   7.600%   $139.6325   $139.63   $139.65   $139.60  
  Purchase #2 $117.58   9.200%   $128.3974   $128.40   $128.40   $128.40  
  Purchase #3 $155.51   8.765%   $169.1405   $169.14   $169.15   $169.10  
  Purchase #4 $10.26   3.865%   $10.6565   $10.66   $10.65   $10.70  
  Purchase #5 $47.38   3.866%   $49.2117   $49.21   $49.20   $49.20  
  Purchase #6 $183.51   13.660%   $208.5775   $208.58   $208.60   $208.60  
  Purchase #7 $122.45   10.500%   $135.3073   $135.31   $135.30   $135.30  
  7 purchase totals = $766.46       $840.9234   $840.93   $840.95   $840.90