I just put up a snap-lock standing seam metal roof and the metal roof manufacturer's specs called for 30# felt as an underlayment. 30# felt has been around for awhile and has a proven track record for use as an underlayment for many different types of roofing material.
If you're asking whether it's okay to lay 30# felt over wet sheeting then that's a different question. I wouldn't want to put felt over wet plywood. That's just me I don't know what the pros would say.