I say go ahead and pick them for display. Glass gem seeds are not that hard to get but, can be expensive compared to other corns. Picking early in my experience drastically decreases viability and really depends on the stage of the cobs. If the plant has started to die back and the husk is starting to yellow you have a better chance of the seed being good. You also can pull up the whole plant and dry it. You can get 50 seed for 5-7 bucks (sometimes less) so is it really worth it to you?