If you're a writer who's ever queried an agent, let me salute you. It's not very fun, I'd imagine. As you've noticed, agents tend to have different submission guidelines and some of us are quite militant about them. I hope you all have spreadsheets to keep everybody straight.
However! I'm here to make your lives slightly easier. While I don't have the power to create a universal submission guideline, there is one thing that 99.9% of agents agree on when it comes to your actual query:
We hate rhetorical questions.
Now, to be fair, some agents don't mind when you begin your query with a rhetorical question. Some just skip it and move on. But no one likes them, which I think it a notable distinction. They're awkward to read, wastes precious query-reading seconds, and can even get you a very quick rejection. Agents read hundreds of queries - sometimes hundreds of them a day (!) - and your rhetorical question is not going to hook us the way a direct, unique description of your book will.
Here's why rhetorical questions fail:
Have you ever wondered... ? Nope.
What would you do if.... ? Whatever your character does.
What if you... ? I'd be living in the premise of your book, whatever that is.
Remember when... ? Maybe, but you shouldn't assume I come from the same background or generation as you.
Do you ever wish... ? Probably not, but hopefully my enjoyment of your novel doesn't depend on my inner desires.
In short, the answer is never a simple "yes." Even if by a miracle you pick the one agent who has been waiting to hear that question all day, chances are he or she will prefer to have heard what your book is about instead. What's worse is that if the answer is a very plain "no" (which it usually is), then all you've done is given us permission to stop reading your query.
You will never be rejected based on a rhetorical question alone, so don't worry if you've already sent out a bunch of queries littered with them. But, for me, if I'm on the fence about a query or I know my reading my pile is getting too large to add to, I may take that rhetorical question as a testament to your writing style. It may not always be fair to the writer in question, but it's an easy way to filter out material when I just don't have the time for new things.
Queries are hard, but there is no magic formula to them either. The only thing agents want to know is what your book is about. Note: your book. Rhetorical questions say nothing specific about you, your story, or your characters. They're like movie taglines, meant to entice a potential audience without giving anything away. Agents, however, are not your potential audience. We're the ones who will help you find your audience. But first, we need to know what your book is about.