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 $17.60   7.703%   $18.9557   $18.96   $18.95   $19.00  
  Purchase #2 $44.35   6.214%   $47.1059   $47.11   $47.10   $47.10  
  Purchase #3 $16.38   3.536%   $16.9592   $16.96   $16.95   $17.00  
  Purchase #4 $164.61   8.367%   $178.3829   $178.38   $178.40   $178.40  
  Purchase #5 $132.26   8.197%   $143.1014   $143.10   $143.10   $143.10  
  Purchase #6 $43.72   13.478%   $49.6126   $49.61   $49.60   $49.60  
  Purchase #7 $180.56   3.393%   $186.6864   $186.69   $186.70   $186.70  
  7 purchase totals = $599.48       $640.8041   $640.81   $640.80   $640.90