They measure tread depths in 32nds of an inch. I believe most tires start with 10/32". The "head of the Lincoln penny" test should put you at 2/32". At that point, the tire wear bars (transverse to tread) should be even with the treads. In the wetter East, that would be getting risky. Of course, poor alignment (bashing curbs and such) will cause the tread to wear sooner and unevenly, meaning you will not get the maximum life out of the tires. Worn struts will also take an additional toll on your tires, thus shortening their useful life.
I believe they have Costco on "the Island" or some facsimile thereof. When you do get new tires at a place like that, they will rotate them for FREE. I take them in for rotations every 5K to 6K and have gotten the estimated tread life out of my tires.
Also, as soon as you get tires:
(1) get the alignment done and watch the ruts/curbs from then on, and
(2) check how your struts are doing (it's easy to test them via the "bounce test")
Costco, unfortunately, neither aligns nor sells suspension components.
When you do go in for rotations, ask them for treadwear readings. They have this little prong that fits in their "pocket protector" and they can give you all the readings in about 2 minutes, that way you can see that you are working your way down from 10/32" in a "proportionate" manner...or at least, you should be.