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 $244.95   11.533%   $273.2001   $273.20   $273.20   $273.20  
  Purchase #2 $98.53   8.872%   $107.2716   $107.27   $107.25   $107.30  
  Purchase #3 $192.91   3.698%   $200.0438   $200.04   $200.05   $200.00  
  Purchase #4 $5.79   8.360%   $6.274   $6.27   $6.25   $6.30  
  Purchase #5 $126.79   7.764%   $136.634   $136.63   $136.65   $136.60  
  Purchase #6 $189.43   6.376%   $201.5081   $201.51   $201.50   $201.50  
  Purchase #7 $13.71   7.114%   $14.6853   $14.69   $14.70   $14.70  
  7 purchase totals = $872.11       $939.6169   $939.61   $939.60   $939.60