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 $158.37   3.609%   $164.0856   $164.09   $164.10   $164.10  
  Purchase #2 $207.86   9.644%   $227.906   $227.91   $227.90   $227.90  
  Purchase #3 $190.41   12.193%   $213.6267   $213.63   $213.65   $213.60  
  Purchase #4 $228.64   8.371%   $247.7795   $247.78   $247.80   $247.80  
  Purchase #5 $17.74   13.518%   $20.1381   $20.14   $20.15   $20.10  
  Purchase #6 $7.86   7.750%   $8.4692   $8.47   $8.45   $8.50  
  Purchase #7 $106.80   9.690%   $117.1489   $117.15   $117.15   $117.10  
  7 purchase totals = $917.68       $999.1540   $999.17   $999.20   $999.10