Best time to cash out?

Been using Prolific for a few months, trying to get some extra money for something in particular that will cost about 100 USD. Right now I have a Balance of £ 39.71 and a Pending Balance of £ 2.90. I was wondering if I should cash out when I have enough to get what I'm saving up for, or I should cash out now. I get a little nervous having earned a bunch and not cashing out, but I think if I wait I'll end up getting a bit more through exchange rates and whatnot.