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 $241.73   13.239%   $273.7326   $273.73   $273.75   $273.70  
  Purchase #2 $11.55   6.119%   $12.2567   $12.26   $12.25   $12.30  
  Purchase #3 $49.55   7.149%   $53.0923   $53.09   $53.10   $53.10  
  Purchase #4 $47.58   4.496%   $49.7192   $49.72   $49.70   $49.70  
  Purchase #5 $173.88   5.388%   $183.2487   $183.25   $183.25   $183.20  
  Purchase #6 $58.71   3.834%   $60.9609   $60.96   $60.95   $61.00  
  Purchase #7 $69.66   10.443%   $76.9346   $76.93   $76.95   $76.90  
  7 purchase totals = $652.66       $709.9450   $709.94   $709.95   $709.90