"Spike camping" is my preferred style of hunting (and I hunt WA also).

Much of what you need depends on the situation. Can you make a fairly easy run for more supplies? If not you will need a fairly stout amount of gear for 5+ days.

Early season water is the biggest factor. I always have a filter, but some areas just have little available, and others have plenty.

Storing gear is not as easy as it sounds, esp. if you will pack it in any length of time before you hunt. Rodents and bears can get into almost anything unless its damn near bullet proof.

What kind of weather do you need to prep for? Where I hunt cold is not usually a factor, but wet certainly is.
In really wet weather even a serious tent and storage is no fun at best and a soggy miserable mess at worst.

I figure I can handle almost any conditions on a bivouac for 1 or 2 nights. After that if things are miserable and wet and I'm suffering from lack of rest I start looking for more amenities, even if that means the seat or back of the truck.

You have to have some plan on getting meat out. I usually have help and/or a game cart.

It is a great way to hunt. Just pick a good area so you don't run into more hunters coming from some other access point than there were where you left from.