Flasks are not airtight, so there is the potential for moisture to leach into the powder. You might be lucky and have dry storage conditions, but why risk it? I always return unused powder to the bottle it came in - this also means that you never get mixed up about what powder you have in the flask. You might use only one powder today, but I am pretty confident that the time will come when you want to try something else, so now is the time to cultivate good habits.
True freedom includes the freedom to make mistakes or do foolish things and bear the consequences.
TANSTAAFL