As usual...there is much miss-information and rumour being passed off as fact.
Coilovers were designed to allow the vehicle to be corner weighted, where each corner takes an equal - or as close to equal weight/load as possible.
They NEED to be at different heights on most cars to achieve this...and should be set up with the car on it's wheels, with all 4 wheels sat on corner weight scales, and the spring platforms adjusted to get equal distribution of weight with the driver on board.
Since most people lower their car so stupidly low, and run it on the bumpstops, all trace of handling has long since gone...so it matters little to most...as long as it looks 'good'.
So they set it by eye, on a flat surface, after the springs have settled.
...and it works just fine.