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 $216.28   12.124%   $242.5018   $242.50   $242.50   $242.50  
  Purchase #2 $34.94   8.605%   $37.9466   $37.95   $37.95   $37.90  
  Purchase #3 $49.69   12.457%   $55.8799   $55.88   $55.90   $55.90  
  Purchase #4 $239.55   8.613%   $260.1824   $260.18   $260.20   $260.20  
  Purchase #5 $66.94   3.426%   $69.2334   $69.23   $69.25   $69.20  
  Purchase #6 $100.63   5.714%   $106.38   $106.38   $106.40   $106.40  
  Purchase #7 $24.97   4.528%   $26.1006   $26.10   $26.10   $26.10  
  7 purchase totals = $733.00       $798.2247   $798.22   $798.30   $798.20