Only if I am going to change the flavor completely.
For example, if I am going to switch to RY4, and have a 555 cart, I don't bother washing. It's a nice complimentary flavor.
However, if you want to wash a cart, heres how I do it.
1. Boil some water.
2. Take off the plastic cap and rubber stopper, rinse off, and set aside.
3. Soak cart in boiling water for 10 minutes or so.
4. Rinse cart under water for a few minutes.
5. Blow out cart
through battery hole for a minute or two.
6. Let sit overnight.
7. (optional) use a hairdryer in the open cart end for half a minute or so.
That's how I do it. I only do a few carts at a time, so the first 5 steps really only takes a few minutes. The blowdryer part just gets rid of any last bits of moisture that may be lying around.