If you're asking your boss's boss because only they will know, if it's by email, will it help to just cc your boss?
I'm probably not a good person to ask...
Pretty much everything is meant to go up and down the ladder with no skipping. But I know a lot of stuff gets filtered out in both directions, because my boss's boss is a micromanaging controlling dickhead.
I am not one to quietly accept whatever comes when they are making inexplicable decisions, so I give feedback directly, which has caused me to have a 1-2-1 with boss's boss, in which I refused to just give in and agree with everything he said - because it's okay to have different opinions, and not agreeing doesn't make it a personal attack.
But also, I have also good relationship with boss's boss's boss, because we have similar mindsets, and he does respond to my many questions - not always positively, and some feedback has been difficult, but useful, and I can trust him, and he is fair. I have done quite a bit to cultivate this relationship over the years, partly because it's protection for me against the toxic relationship between my boss and his, but mostly because I like and respect him, and there are some things he does which are amazing. And it's okay to disagree with him. He does pull rank occasionally and point out I need to discuss something with my manager, but it's good.
I have also spoken to his manager (4 ranks up), and today, sent a query to his manager (5 ranks up)... and I once sent a mail to the CEO (8 ranks above), giving feedback on something he'd said in a company-wide broadcast. I'm on first name terms with the head of HR in EMEA, too.
I am not that bothered about hierarchy - if someone is the one who will know the answer to a question I have, I ask it. I don't think I'm normal, though - most of my peers can't even name our entire reporting line, let alone have corresponded with them.
I can see why some people get pissed off with me sometime's, but we'll all survive. I'm probably not a good example to follow, though.