C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 18:
[18.3] Should I try to get things const correct "sooner" or "later"?

At the very, very, very beginning.

Back-patching const correctness results in a snowball effect: every const you add "over here" requires four more to be added "over there."