Cap them.
There are a number of reasons for this, but first consider the nature of a cartomizer is such that liquid is held in a matrix with an immense surface area. Not all of that surface area is exposed to air, but a lot more than you'd think. So air can really get to the content.
Some
e-liquid has some water content to facilitate flow. If the humidity is less than 100%, over time that water will evaporate. That's problem one. Not all e-liquid contains water, though -- but there are other problems.
If the e-liquid contains no water, it will actually absorb water and dilute the juice if the humidity is very high at all.
The biggest problems are oxidation and volatilization.
I don't really know what flavoring compounds are used. Some will only oxidize at all at quite high temperatures; but others may oxidize a little at a time at lower temperatures; and this oxidation will hurt the flavor.
Think of it like wine. Wine is really a very complex substance -- that old equation from high school where the sugar gets converted to ethyl alcohol only tells a tiny part of the story. Also created are a variety of higher alcohols, ketones, aldehydes and esters in a mix that will vary depending upon the pH of the starting juice, the temperature of fermentation, the time that has elapsed and more. Wine actually changes chemically over time. But one thing practically all authorities agree upon with the exception of a couple of unique varieties is that exposure to air (even if it is filtered to keep out microbes) hurts the flavor of wine by disrupting these substances through oxidation.
The flavorings used in a cartomizer are similar -- they are likely composed of a good many substances of that nature that are overall reasonably stable but that when exposed to oxygen over a long period of time will change. Thus the flavor will change.
Nicotine will also oxidize which adversely affects its effectiveness.
Then there is the fact that even though something might not theoretically vaporize until it reaches a certain temperature, minute amounts will vaporize at lower temperatures. Thus, you will lose some flavoring components over time.
So I say that if you are going to play, be safe and use the condom. The flavor and effectiveness will be much improved!
