Exact Timing Constraints vs. Over-Constraining

I have seen much conflicting advice w.r.t timing constraints on this
newsgroup, and I was hoping that the proponents of both camps might
make explicit their reasons so that others (meaning I ;) can benefit
from their experience.

I have seen many posts recommending over-constraining the timing
requirements in an effort to ensure correct functioning in the
presence of uncertainty and unknowns, but I have also seen many posts
stating that the tools are now good enough that this is no longer
necessary, and that giving the true timing constraints is sufficient,
and will allow more latitude for the tools to put their effort where
it is truly needed.  I doubt if it's so cut-and-dry, but which is the
"right" one?  Or have I completely misunderstood the problem?

Thanks for your time,
Pierre-Olivier

--To reply directly, remove the obvious in
[EMAIL PROTECTED]