If you're buying a demo/showroom car - then you're probably better off buying late in the quarter vs. early in the quarter.
And if you wait too long (which could be hours or days), especially as it gets closer to the end of the month, you'll likely find the car you've been looking at has been sold.
As for purchasing new Model S cars - if you ordered a new one today, it would have some improvements for a 2017 version (interior trim changes, MCU 2, …) - and that will always be true, that longer you wait, the more likely there will be changes. Though, most of the changes are minimal - since introducing the 100 battery packs, Tesla hasn't made any large changes to S or X. And with the focus on making more Model 3's, it seems likely we won't see major changes for a while for S or X.
Musk indicated the V3 supercharging has been pushed out at least 6 more months. It's possible we'll see a new battery pack/charging system for S/X to take advantage of faster V3 supercharging - but until Tesla provides more information, we're all just guessing...