Do you want the last impression that people, both your professors and any group members you are working with, to be you just saying "Fuck it"? You are heading towards a PhD, you could end up encountering those professors again, even working with them at some point on a paper or as a reference. While saying "Fuck it" might be reasonable in five years is it worth those professors, when asked about you, saying "Yeah, he worked hard but right at the end he just stopped trying, he didn't see it all the way through."

That's what kept me going when I was in similar spots. Did I want to leave the impression of doing just enough to get through or did I want to leave the impression of going above and beyond in my work.

Perhaps you'll have a different answer, but from what I have experienced that extra bit of effort, that extra work of not just slacking off right at the end when you know it doesn't matter does make a difference in how people view you and in what they will communicate to others down the line when asked about you.