Used them for years. I would compress them to the point that the fluid would reach the top and slightly overflow and then I closed the cap. Never had a problem with air getting in and could keep developer for 3 or 4 months. Maybe longer but I would dump and mix fresh. As long as you have fluid in them I think rinsing is all you need to clean and I would always refill with the same developer. Never used for fix or anything else.
When in use, they will not stand straight but its not a problem. No longer use as I think amber glass is best and use 250ml filled to the brim and use one shot. Safer and superior than using plastic food bottles IMHO. I think in trying to keep c41 this is especially important.