This definitely varies by state...and by time. And by price. And by option.
Not long ago in WA, EVs were exempted from sales tax. So were batteries and charging equipment. So, aside from those, it was cheaper to buy an option with the car rather than later.
This is still possible. Say the new LEAF is $25k and there's a $3k option (that is not battery or charging related). Sales tax would be waived on all $28k if you purchased it with the new car. But if you added the $3k option later, you would have to pay sales tax on it.
HOWEVER, under current rules, the sales tax waiver is limited to $3,200, which is covered by the Model 3 base price. So this doesn't apply to any Model 3 options, because the sales tax
for the option won't be waived on the purchase, so you'll have to pay it no matter when you buy it. (And, any car with a base price over $42,500 doesn't get any sales tax waived, so nothing applies to S and X buyers).
And just to make it more fun, note that the whole WA sales tax waiver will expire soon, probably in early 2018, although it is hard to get numbers from the state to figure out exactly when it will happen (there is another thread on that).
Whew. All of this is just for WA; those of you outside of WA are on your own deciphering the local laws.