I have split views on this:
If you want/need now then buy now. There will always be better products along in the future, this applies to so many things, not just technology. I also agree that if you feel an Air meets your needs now then why should it not meet your needs a day later just because a new product has been announced. Answer: It shouldn't.
However:
Apple always brings out better a featured product for the same price or better featured product for less, rarely do they bring out a better featured product which costs more than the product it replaced.
So if your current computer is not broken (i.e. you are upgrading) then you can normally hang on another week or so. It is not a matter of urgency that you have new machine right away.
It's a little frustrating knowing that for the exact same price of the model you just brought you could have brought a better model had you waited a week. This is doubly galling if you could have quite easily waited that one extra week.
The other thing is that Apple computers when launched are the best value you will get them at, and Apple normally takes a hit on the margins to achieve this.
As the product continues on its life cycle, Apple will rarely (practically never!) reduce the price, so as every day passes the value for money on the product reduces (as the cost of components reduce over time).
This means that shortly before a new revision the product likely represents the poor value for money over when it was originally introduced. This is of course where Apple will pick up the margins they lost early on in the products life. Hence one of the reasons why Apple rarely pre announce anything.
On balance I would say that Apple has that few major events now that it is probably prudent to make a note of when they are in the year.
I'm sorry that was a bit clumsily explained, succinctity is a skill I am still trying to master, hopefully it makes sense though!