the best time to get a car is when you need it. For instance, Volt's already lease for more than 10% off MSRP and can find volts that sell for even more than off MSRP (seen sale prices as low as $28K for brand new 2017 volts). so if a 20% sale gets you to a 27K sale price, you'll save $1K but have possibly been out a car for months that you might have benefited it. As farm bureau is going away (see
https://www.fbadvantage.com/gm), that brings down that savings to a possible $500. If buying 2 months early gives you $250 in benefit (I'd argue possibly more if you do need the car), you benefit.
On the flip side, if you don't need the car, getting a deal that saves you only $1 or $2K isn't something to kill yourself over. Yea, if it was like the march leases where with an asian lease you were practically being paid to take the car, sure, but that's generally not the case, and in those cases the longer you wait, the more valuable the car to money ratio will be to you when you finally do buy.
just my 2c.