Does the finals always to be the proving ground for your final grade?

Based from your experience, does the finals always the proving ground for your final grade especially between A and B?

Your cumulative grade throughout the semester
Week 1 - 100%
Week 2 - 95%
Week 3 - 92.6%
.
.
Week 15 - 90%
Grade after finals = either A or B???

Or do you usually get A along the way and do not worry about the finals because even you get B on finals, you will still get A on the final grade?

It depends on how your professor calculates your grade. Your syllabus should spell this out. Some professors may weight the final a great deal, e.g. 25-50%; others may weight it much less and so the final would not have too much impact. I would never assume that your final exam grade would not have an impact on your overall grade.