In reply to cgales:
Unfortunately, we live in a world that is driven by timelines and schedules. Where designers will make huge number of changes and many a times due to other than verification inputs. FOr example, physical design feedback.
Anyway, let me articulate through an example of what I am saying
if a = b +c
d = e + f
y = a + d then checking y as expected should be fine enough.
I may not want to worry about whether a and d are as per the expectations.