Exactly. The IRS gets your money throughout the year one way or another. Whether that be through W2 withholding or estimated taxes. It's up to you to have the proper amount withheld. If you have too much withheld (i.e. you are expecting a $7500 credit but you don't account for it) you are giving the government an interest free loan. If you have too little withheld, you have to pay a penalty. If you have just the right amount withheld, you win.
I had my withholdings adjusted for 2015 for the LEAF I purchased in March, so it will be easy for me to just leave everything the same to take the credit again in 2016 for Model X.