I think it depends a lot on where the LEGO name is used. On site-specific content (tags definitions come to mind, for example), it's probably best to make sure LEGO is written as intented by LEGO itself.
Let's keep in mind that we will1 attract the attention of LEGO and that they'll appreciate the effort. Let's make sure from the start that they consider us as a valid and interesting online community, and not as a thorn in the foot (or pain elsewhere).
In short, let's play nice with them, and they'll play nice with us.
Of course, it's not realistic to ask for all posts to be written according to LEGO guidelines, and it's certainly not the goal that LEGO has any form of control on the site either. So I'd say we can encourage people to follow the guidelines, but we can't really force anyone.
There's one particular point of their guidelines which I think can't realistically be enforced, which is to always use LEGO as adjective. While it's usually not a big issue, sometimes it's just too cumbersome to forcefully add a noun. I'm fine with anyone saying "I play with my LEGO in my room".
As for editing posts to respect the guidelines, honestly, I don't see any harm in doing it. On languages SE sites for example, editing spelling mistakes away is common practice and it doesn't bother anyone. Heck, even spaces get edited away on the French SE.
But anyway, all this is probably going to be discussed on the meta site when it's up.
1. Probably because I'll inform them of it myself, on the ground that it's better they know about the site from the start rather than having to change stuff later on in the game.