I havent had the honor of yet appearing for CSS interview. But for other interviews, public and private that I have appeared in, always there has been a panel.

So following your question, if we assume that we have to shake the hand, that brings up new questions: whose hand to shake first? which side to start from? do you start from the right (Islamic teaching) or shake hands with the incharge of the interview panel?

Therefore, I have always been advised to not approach the panelists and shake their hands. Now that this point is under discussion, another thing I noted was that in all cases of interview, the chair for interviewee (me) was placed a good 2 -3 feet away from the table of the panel. This in itself, to me, indicated not to go forward towards the panelists and shake their hands.
I think it goes without saying that should the panelists offer you their hand for a shake, then do shake their hand.

Secondly, coming to taking the seat: do NOT sit down unless given permission. In all interviews I have had, after entering the room, I said "Salam" loudly, then approached the chair and stood by its side. After that I asked "May I have seat please?" and got the answer. In all interviews of public sector, I was not offered the seat automatically, I had to ask, whereas the private sector interviewers offered me a seat.

May Allah ta'ala help us all reach our goals, ameen. Good luck all.