Lots of people have touched on the fact that liquid might be altered by its time in the tank. But it really depends on what kind of liquid you had - some, with lots of artificial sweeteners, will darken and the flavor will change significantly due to exposure to heat from the coil and light from hanging out in your clearomizer. Others, not so much...do a visual check before draining your tank - if the liquid isn't noticeably altered, you can probably pour it back into the bottle. If it's considerably darker, think about pouring it into a bottle of its own, vaping it through to the end, or just dumping it out...
