Depends on the individual's circumstances.
I can modify a car from day one without worry because I can repair it if it breaks.
Some people don't have the means to do so and wait until the warranty is up.
The way I see it, if you have to keep the car running for your livelihood, then make sure you wait until the warranty is up and you have another car to drive if the one you are modifying is broken.
If not, do whatever you want, whenever you want.
I can modify a car from day one without worry because I can repair it if it breaks.
Some people don't have the means to do so and wait until the warranty is up.
The way I see it, if you have to keep the car running for your livelihood, then make sure you wait until the warranty is up and you have another car to drive if the one you are modifying is broken.
If not, do whatever you want, whenever you want.