Does LG / Sprint care about phone rooting at all? I have heard it no longer voids warranty

They replaced the Mainboard of my rooted G4 a year ago without even a comment.
On the other hand, it was a known hardware problem for that model, devices built in the first few months after Launch were likely to suddenly brick themselves...
(LG directly, no idea about Sprint)

I'm pretty sure rooting doesn't void warranty any more. Companies are simply not under any obligation to service your device if you do root it.

Nonetheless, I convinced Samsung to replace the mainboard on my Galaxy S7 Edge for free under Warranty, after bricking it by accidentally flashing over a locked bootloader :D