No, the contents of the engram is determined the moment you pick it up. Well I am not sure the item itself is, but its level is. So if you are level 10 and you obtain one, saving it until you are level 20 will not make the contents of it more valuable. Holding onto engrams for this reason will just be a waste of space. It is far more use to decrypt them as soon as you obtain them so you can take advantage of the contents. Holding onto them won't increase the value and by the time you do decrypt them the contents will be a low level and useless to you now that you are at a max level.