The Importance Of Final Year Results In US PhD admissions
Admissions to US graduate programs are generally conditional on final-semester grades. That is, you are required to pass your classes, and in general maintain a reasonable GPA in them. Developing a case of "senioritis" and getting D's on everything could have an effect on your admissions offers. (Whether or not it will depends on the department and the school.)

However, the grades earned in the final semester do not influence the decision whether or not to accept you in the first place; that is determined by your grades up through the end of the first semester of your final year (or the last semester of your penultimate year, if the grades aren't available by mid-year).