I always go to the table knowing what I can get from my own bank/credit union/whatever.
But I also look for convenience. If the dealer can get close to the rate that my bank can give me, it's easier (in my opinion) to get financing through the dealer rather than have to make trips between the dealer and your bank, etc.
Do the math with several different loan rates on the principle you plan on borrowing. You might find that the difference of a quarter of a percentage point may only cost you a dollar or two in your monthly payment.