Although I don't know of an actual explicit rule reference for or against banking, I'd allow it under the same restrictions as retraining: once you take the feat, you have to stick to the restrictions applicable to your character when the feat 'slot' was gained.
Example: if you get a new feat at lvl 3, but 'bank' it, and fill it in at lvl 6, you can only use it to take the feats you were allowed to take at lvl 3. If the feat has a requirement you didn't meet at that lvl (skill ranks, minimum caster level, other feat) than you can't take it.