mrstat

Not nearly as important as the letters for Stat PhD programs. Since most Masters programs are unfunded, they will admit most students who meet the minimum coursework, GPA, and general GRE requirements.

With a 3.7+ from UC Berkeley and grades of B or better in your math/stat classes, I imagine you will not have any difficulty getting into most Statistics Masters programs. 

As you said clearly nobody will look down on you for pass/fail grades given the circumstances -- but if you are going to get straight A's (or at least A's in your stats classes) and improve your GPA, you should obviously do that.  If you are going to get A-s or below, and thus not improve your grades, maybe you'd be happier relaxing?

FWIW I also did my undergrad in stats at Cal a couple years ago and had a similar (but slightly worse) gpa. I applied to Stat PhDs this cycle and ended up getting into a couple of top 40 programs, with the caveat that they were big state schools so were easier to get into. Our LoR are obviously going to be different, as well as the rest of our resumes, so I don't know how seriously you should take this data point but this is just to say that the gpa you have now is definitely not terrible.
One potentially important point though is which classes you're taking. I'm not sure about masters but PhD programs really care about real analysis (or similar advanced math class). I have no idea how admissions committees will compare someone who did P/NP for real analysis during the crisis vs. someone who got an A or A-. The cynical part of me thinks that there must be at least a couple of top programs that will overtly prefer the latter over the former, but I'm obviously just speculating. But if masters programs don't value real analysis that much (or you're not taking it) then this is a moot point.

I think since most  master's programs aren't extremely competitive, and you're coming from a good school with a decent gpa, you'll be fine.  Since you're paying for the degree generally and not doing research, letters are much less important than for a PhD.  Just some professors that you did fine in their class and maybe a future boss will be fine.  There doesn't seem to be a whole lot of upside to getting the letter grades given that you're not sure you'll improve your GPA. I'd personally take the free opportunity to take all classes pass/fail without raising eyebrows.