I reuse cartos all the time, when I find a flavor I really just love it get's a dedicated carto that I mark with silver paint, but when rededicating cartos I normally clean them out by:
Taking a turkey flavor injector syringe, without the needle, some 7/16th vinyl tubing, and boiling water. Attach the tube to the syringe, dipping the other side into the boiling water (to soften it up) and sliding a carto bung side first into the softened tube (bung removed). Then I proceed to flush boiling water through the carto for a minute or so, and use a piece of tubing, threaded with string, to spin dry the carto (think David and Goliath style). Let everything sit for a bit, refill, and enjoy my new flavor.
All that said, I do use an ego tank to sample most flavors, and if I'm on a carto kick I usually wait till I need 4 or more cleaned before I bust out the cleaning kit mentioned above. Easiest way to sample is always going to be dripping or dipping, cleanup isn't even an issue when going this rout because you can just drip some VG/PG to clean the taste out before going on to the next flavor.
Hope something here helps you.
Edit:
If you're going to use isopropyl alcohol for cleaning, make sure you let every thing dry completely as isopropyl is unsafe to inhale. I've seen recomendations for atty cleaning with isopropyl that recommend letting the unit set and dry for up to 48 hours, just to be safe.