Everyone seems to find their own balance with supplements, and for me the general rule of thumb is to try to find some science to back up any claims for a particular supplement (I Google the name of the supplement along with a key word like "study"), talk it over with my onc to make sure it's going to be complementary with whatever treatment regimen I'm on, and then try it for a month or so. I don't take a lot of anti-oxidants while on chemo, but when in between chemo I tend to do more Vit.C and CoQ-10 for sure.
I was taking TONS of supplements and making fresh juices from veggies (the whole nine yards) after finishing treatment for primary disease, and I still recurred 6 months later. So in my experience it didn't stop recurrence, however, I still believe a strong immune system is crucial for good quality of life so I do what seems sensible and just try to eat a balance diet full of good quality protein and fresh veggies. Also the occasional bowl of ice cream or bar of chocolate
<3 Lolly