I don't know how much truth there is to this, but I have heard that it's better to leave it in the glass vial or amp as the plastic in a syringe is porous. The pores can, overtime, start to absorb some of the compound thereby lowering the potency. I remember reading about it a long time ago on another board. It probably wouldn't make much of a difference but I think you're still better off drawing just when you need it.