On my Y, I did the entire front including fenders and A pillars. On my S, I did the entire car. On my 3, I did nothing. The Y I always regretted not doing the entire car and the person who bought it did the rest of the car. All my cars are white.
I did the entire S because I organized a group buy and it was worth it. If pay 6-7k to do it, it is definitely not worth doing it, especially on a cheaper car like the Y/3. On my S it has helped a lot. If I had just done high wear areas, on the white I'd have a seam of dirt and that would annoy me a lot even though I am not OCD.
The Y with its massive snout was saved from a huge beating based on the impacts of the PPF. The S has seen a lot of highway miles and based on the windshield glass, has had its nose well protected by the PPF.
I hate to say it but the 3 is more of a beater car. It is a daily driver and sees more in town use than highway and the paint is holding up pretty well on it. I'd likely never PPF a Tesla again to be totally honest unless it was a Roadster. Unless you have OCD, or get a serious deal on PPF or do it yourself, I don't think it is likely worth it for most people's situation.
So for me, under 5k on an S/X it is worth it. On a 3/Y, any stone chips aren't likely to devalue the car enough to really matter. Same applies to the S/X but the pristine finish would help move the car faster for picky buyers with more disposable income. I'd also add that mudflaps, even though less than great looking, are probably the best bang for the buck.
If doing hard launches, the flaps and PPF where rocks get kicked up will help a lot. YMMV.
I agonized way too much over this when I bought my first Tesla. Yes, the paint isn't as good as any of my other cars but PPF is likely overkill. I am very happy with who did my PPF install and tinting in Dallas.