I believe yew is the only wood you really need to worry about. I have been checking staves I keep in my garage all year. Even in summer when the temps are way high and the RH is way low nothing got much below optimum MC. Like I said, yew was the exception, it got down to 6%. Make sure you keep them inside or in the shade when you aren't using them. If they get left in the summer sun, they will bake dry and they will crack even without stringing.