Some master cylinders have a weep hole that can get hydraulic fluid on your bike and other stuff nearby (and strip the paint, if it's DOT fluid) if they decide to weep while hung up. And if there's gas in the reservoir, yeah, it may get into the MC and the hose if you hang the bike. So I'd recommend keeping it wheels-down. If you have to hang it, try not to stroke the brake levers while it's hung up.

Keeping the fork seals lubed is technically a perk to having the bike upside down or front-wheel-up, but the reality is they're so well sealed it really isn't an issue in the first place. Normal operation will be plenty on that front.