Is now a good time to buy a Mac?

No.

The Retina MacBook Pro was refreshed and is likely to be updated in a few months.

The Macbook Air was refreshed and will probably never be updated, so stop asking.

The Mac Mini was refreshed and nobody has a clue about this one yet.

Unless...

You want a Macbook, which was refreshed so now's the right time.

Why shouldn't I buy?

Apple rarely discounts hardware, so you'll pay this year's prices for last year's hardware. It's not worth it, if there's an update on the horizon. Don't be an idiot, just wait.