I am level 23 and I have still not had a purple (legendary) engram drop but I am thinking when I do finally get one I should hold on to it until I level my Cryptarch up to hopefully increase the chance of it decoding into a Legendary Item. My Cryptarch is currently only level 4.
I have read online lots of people disagreeing with each other on this issue, some say the item is determined at time of pick up and not at time of decryption (why bother with the Cryptarch at all in this case) other people are saying they never get greens (uncommon) at higher Cryptarch levels (27+).
Has anyone figured out a conclusive answer to this issue or tested it with significant numbers of engrams?
Edit: Since the latest patch this question has become far less pertinent as engrams never decode lower than their current level.