How rigid is your design specification?

At the start of any major project there is typically a design specification.  As the project progresses new requirements naturally emerge. My questions are,

Once the original design specification is determined how rigid do you or your team adhere to that spec.?

How do you manage the balance between time to market and the never ending release date?

And where do you draw the line on changes that are significant to a customers needs and version x wish list items?

Thanks,
Ron