(This method is written by Floyd Sense, I have had good results with it.)

            Use putty only if the chip is in an inconspicuous place and you'll be
            using a toner spray over it.  You could then hide it pretty well.  If it can
            be easily seen, I'd go with replacement veneer.  Be aware that veneer
            comes in more than one thickness and of course in any kind of wood you want.
            You'll want to match both the wood type and thickness.  I've done some patches
            that are practically invisible after the cabinet is refinished.  You should
            always cut the veneer patches at an angle.  Cuts at right angles to the
            grain will always be visible.

            So, say you have a chip in the veneer that's maybe 1/4 by 1/2" and it's
            on the front or rear edge of the cabinet.  You would first cut the edges
            of a triangle shaped area, which includes the 1/4 by 1/2 inch area.  The
            base of the triangle would be the edge of the cabinet, so you've made the
            veneer cuts at an angle across the grain.  You then carefully remove the upper
            layer of veneer in the triangular area with a pen knife.  Next, choose
            a matching thickness of veneer and cut it to fit the triangle shaped area
            you just cut out.   To get the angle right, trim with a razor blade and
            straight edge until the patch can be slid in tightly.  Then, mark the edge on
            the patch piece and cut it to that dimension with the razor blade.  You can
            now use a contact cement or a wood glue to glue the patch in place.  You
            want to make sure to apply pressure across the entire area of the piece.  For
            small patches on a curved surface,  I use a steel ruler, bent around the bend
            over the piece and secured with clamps.

            Don't try to start with a piece of veneer that's too thick and then try
            to sand it down flush after you've glued it down.  You'll have a hard time
            hiding the change that the sanding makes to the surrounding wood.  I
            know one fellow who obtains all of his patch veneer pieces from junker
            wooden cabinets.  He lets them sit out in the sun and rain until the veneer
            separates, and then he cuts the upper layer up into pieces, discarding
            damaged areas. That gets you some real good veneer, of typical
            thicknesses, for just about nothing!