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 $240.89   13.535%   $273.4945   $273.49   $273.50   $273.50  
  Purchase #2 $105.99   13.463%   $120.2594   $120.26   $120.25   $120.30  
  Purchase #3 $148.75   10.504%   $164.3747   $164.37   $164.35   $164.40  
  Purchase #4 $27.30   5.523%   $28.8078   $28.81   $28.80   $28.80  
  Purchase #5 $201.83   11.805%   $225.656   $225.66   $225.65   $225.70  
  Purchase #6 $153.27   13.116%   $173.3729   $173.37   $173.35   $173.40  
  Purchase #7 $77.25   5.350%   $81.3829   $81.38   $81.40   $81.40  
  7 purchase totals = $955.28       $1,067.3482   $1,067.34   $1,067.30   $1,067.50