Don't have to be member at time of travel
I've been wondering this for years and the FAQ are silent but I clicked autorenew when I joined and there's no unautorenew option (classic .bomb).
Anyways, this August I was told that my payment method had expired (it hadn't) so I decided to let my membership lapse for a few weeks. Flew an old itin and got my 5 bucks! Reupped before booking any more tickets.
Nice way to stretch out your membership if you don't book often or are exceedingly cheap (both of which apply to me).