I used to clean them and then store them with thinner in them.. Its actually worse.
After cutting corners on gun cleaning so many times over the years and getting frustrated and throwing guns in the trash, I have gotten downright anal about gun cleaning anymore. I actually pull the cup/needle/cap/tip between stages and sometimes between loads and clean/dry them individually. I just do it while the material is flashing, it only takes 5 minutes working fast. I typically clean everything with lacquer thinner, then a shot of Naked Gun or the SEM cleaner, then hit it with a air blast from the air nozzle to dry it.
I store them that way. I found when you leave thinners and reducers in there, that thinners and reducers have "stuff" in them... Over time the reducer/thinner does evaporate, leaving "stuff" behind.
I have shelves in the shop made from 2x4's and plywood. I keep my guns on the vertical 2x4's, using two stragically placed nails that keeps the guns perfectly vertical.