are you saying its better to wait for a few days? or that it's just fine to fill em ahead of time?
also what's our method of just tasting a juice? do u fill one up and hope for the best?
You can begin vaping on a cartomizer immediately after filling it, especially if you use the method I use to fill them.
I first put a toothpick into the center air hole to keep e-liquid from entering it. Then I add liquid to the filler material as I slowly rotate the cartomizer between my fingers. When the filler looks soaked like a slushy drink, I take it and shake it like one would do with a thermometer. This forces juice down deeper into the filler much like a centrifuge would.
I continue to add more liquid, and then shake it down again. When the filler will no longer accept more juice, I remove the toothpick and add a drip tip. It's ready to vape.
If you use the other popular method of filling a cartomizer (the condom method), you will need to blow out excess liquid out of the cartomizer air hole (and risk getting juice into your mouth) and still need to let the carto "sit" for a period of time for the filler to fully absorb the juice.
With the method I described first, there is no need to blow out the carto hole (because of the toothpick) and no need to let it sit (the thermometer shake technique takes care of that part).
It's perfectly fine to fill up your cartos ahead of time if they will not be used for 2 - 4 days. The method of capping the cartos with the plastic condoms that empty cartomizers come shipped with work great to keep the filled cartos from drying out, as RedhatPat suggested.
Many vapors use a dripping atomizer to test flavors. This is a carto-like device, but contains no filler material; just an empty tube with the heating coil. Since there is no filler to contain the juice, it can only use a few drops of e-liquid at a time. You add a few drops, vape a few times, and then add more drops if you wish to continue vaping with that flavor.


dripping atomizers
An alternative to an atomizer or dripping would be a Nano clearomizer. This is a tiny clearomizer that only holds 1 ml of juice, can be easily filled by removing the mouthpiece, and used repeatedly by removing one flavor and adding another.

Nano clearomizer