The yucky taste comes from water that is still in the carto is you leave it to dry on it's own for a few days. Imagine leaving a glass of water on the table for a few days, and then taking a drink out of it.
Tap water contains a lot of minerals that will be left behind with evaporative drying (oven, dehydrator, bag of rice). You can alleviate this by doing a quick rinse with distilled water after you've washed your cartos.
The concept of the oven method is that it should be hot enough to turn the water into steam, but not so hot as to destroy the materials the carto is made of. 220 degrees should be perfect, since 212 is the boiling point of water.
Ideally, a convection oven would work best, since the moving air would move the steam away from the innards of the carto once the water has turned to vapor.
I have had good luck soaking in vodka/grain alcohol for a few hours, drain. Boiling in distilled water for 15 min, drain. Re-boil in distilled water for another 15. Cook at around 220 degrees for about 1 1/2 hours and then just leave them in the hot oven overnight. Seems to work for me.