First winter resting a dendrobium in S/H, is rainwater ok or should I switch to RO?

[bateaux_mousse]

I was just reading in another thread that resting dendrobiums should get zero nutrients. It got me thinking that even without added ferts the rainwater I collect passes over my roof and whatever's in my gutters, and has to be picking up some nutrients from the decaying leaves. Should that be insignificant enough that I can keep using it, or should I switch to RO? Or maybe just run my rainwater through a filter?