It should be based entirely on ideals. We have ideals of democracy, human rights, and Constitution that we attempt to base our whole system of government and society on. If we abandon any of these things in order to make ourselves more "secure", what exactly is it that we're securing? The principles need to be protected too.
This will obtain a fairly interesting batch of responses. For my own bet, I would say that America's ideals should play a very strong role in national security. I don't think that the protection of American national security trades off with our beliefs and the ideals to which our nation has been committed for over 200 plus years. Keeping America safe can be done without negotiating our principles and ideals. Since the September 11 attacks, there has been considerable analysis of how American values can be maintained and how safety can be a critical component of the new battles waged against terrorism and those who wish to do the nation harm. In the end, it seems to me that if American values and ideals is the primary foundation of what strikes fears in the heart of those who wish to do the nation harm. In sacrificing these ideals, it seems as if the terrorists, at that point, would have already won.
This depends on your point of view. The idealists say that national security policy should be based on our values. This has been tried many times. Wilson tried to do this with his 14 Points. Bush junior tried to do this with making the Middle East become more democratic.
However, the realists believe that we should be guided only by our national interests. We have acted on these lines when we have allied ourselves with dicatators or with racist regimes like the South Africans (during apartheid) because we thought they could help us be more secure.