I also disassemble and wash my mask at the same time without the headgear. I would not wash a mask with foam parts this way. I have a spare mask and let it sit dry for a week as well.
I've done it this way since 2004. The tanks stay squeaky clean, and none of my tanks seem any worse for the wear. My masks eventually get a little grungy looking, but I probably get a couple of years before I need to replace the cushions on my ComfortGel masks.
You can get the ResMed cleanable tanks online for less than $40. I would expect them to last several years.
Many of us who use distilled water tend to top them off every morning with no obvious problems. I do disassemble and clean it weekly.