Posted as answer by request of @BaffledCook:
Here's an slightly informal blog post outlining changes in taste between varying degrees of freshness in the grind of a coffee.
http://investigationsblog.wordpress.com/2010/03/03/do-i-really-have-to-grind-coffee-right-before-brewing-it/
The short version is that the coffee starts losing freshness as soon as it is ROASTED. The longer it sits the faster it goes stale. The more surface area it has (ie: you've ground it up; also, the finer the grind), the faster it goes stale. The more you expose it to air, the faster it goes stale. So, seal your coffee in an air tight container at room temperature. And grind as close to the time you add water to it as possible.
How much of a difference detected depends on the the actual coffee been, the roast, the taster's taste buds. If you buy a bean that's been sitting on the shelf for 3 months already, you will probably notice less of a difference than a bean that was roasted last week. I buy beans that are roasted and sold within a week, and I DO notice a difference if I leave the grounds for a day or two before drinking.
They will technically last a long time in the freezer - up to many months - but this is not the best way to store them as they lose flavor quickly (as little as 5-6 uses). Even though the beans will last, storing them in the freezer is not ideal. It affects flavor for a number of reasons. (See below)
If you haven't opened the bag, store the bag in the cupboard, away from sunlight, heat, and excess humidity.
The best way to store coffee beans after being opened is in an opaque, airtight container at room temperature (unless you have a resealable bag with a degassing valve designed for storing coffee). After being opened, the beans are considered fresh for 2-3 weeks, although if you are strict about the storage you may be able to get up to 5 out of them. (When I say fresh, I mean with minimal flavor change/loss)
Why the freezer is bad:
Freezing beans severely lowers the humidity in the bag or container. This causes the beans to become stale very quickly. In effect, you are slowly freeze drying them. Also using cold beans in a grinder can mess with grind due to the oil on the beans being in a more solid state, which in turn affects the brewing.
If you do decide to store them in the freezer, put them in an airtight container and when you go to use the beans let the container warm up to room temperature before opening. This way you do not mess with the overall humidity content of the container too much. Especially let them warm up to room temperature before grinding.
Sources:
I am a Barista in a cafe and a lover of coffee at home. I have tried both methods of coffee storage and have dealt with many customers who have tried both.
In my store we follow these regulations:
- Beans that go into an opaque container that is not totally airtight are fresh for two weeks max.
- Packaged beans with a degassing valve are fresh for six months or longer in the bag.
- Beans, once ground, cannot be used after two hours (that being said, ground coffee starts losing flavor immediately).
Obviously, the sooner you use everything the better, but we follow those guidelines.
Best Answer
It is NOT necessary to store coffee in the freezer.
It can be helpful if you want to store it for long periods - i.e. months.
Ground coffee should NOT be thawed and re-frozen, nor opened frequently and put-back in the freezer. Think of your freezer as long-term storage, and take out a week's worth of coffee at a time.
Also, it's not going to make a huge difference either way - if you want better tasting coffee the best bang-for-buck would be buying whole-bean coffee and grinding it as you need it.
Here's an obsessively detailed taste-testing account:
http://www.home-barista.com/store-coffee-in-freezer-conclusions.html