1. Yes, you can ignore and carry concealed - in most states you will still be asked to leave and given an opportunity to do so, before being charged with trespass. Always claim that you never saw the sign. This won't even come up anyway unless you get made.
2. So that's the immediate practical issue. But the bigger, longterm issue is that is DOES matter, most definitely. Having those "stupid signs" conditions the sheeple that guns are bad, period - and that we have no right to BEAR arms. This is about long-term mental conditioning, which can affect people's views and the way they vote. So CostCo MUST BE boycotted and bombarded with correspondence, by all means.
3. WA is the new gunkid, IMO.