If you haven't activated the approach, it won't always, or even most of the time, activate itself. You might find yourself accelerating, fully configured, at TOGA thrust.
90% N1, on the A320, will assure a safe airspeed (assuming you're not approaching a stall). While the appropriate power setting, based on configuration, weight, etc., will likely be a bit lower, 90% will keep you flying and avoid a stall until you stabilize the aircraft and adjust power to the appropriate setting. Figure that out later, when you have the time. If you don't know your airspeed, and your protections are suspect, erring on the high side, I think, is wise.