Biking Etiquette Question

You're very thoughtful to ask ...

It's polite to ask "May I sit in" (that is, draft). That lets people know you know cycling (at least the lingo) and can probably safely draft.

But I don't think it's strictly necessary. I've had all sorts of people catch easy rides on my tail (wait, that came out REALLY wrong) without saying anything ... which is really bizarre considering how slowly I cycle. It's kind of a compliment.

Otherwise, yeah, 3-4 bike lengths is wise and safe.

IMO, 3-4 bike lengths is perfect. It's also good practice for legal drafting in races. Once someone passes get in their draft, start counting seconds, and rest while slowly backing off a a rate that you end up 3 lengths behind in exactly 15 seconds.

I do not think it's polite to jump on someone's wheel and stay there unless you ask. Since I generally pass quite a few folks in training, sometimes the better riders want to pretend like they're bike racers and jump on my wheel. When they do I just stop pedaling until they must pass me. Once I'm rested up I blow by them so fast they can't get on my wheel.

It's not personal and I'm perfectly comfortable riding in a tight group with good riders if I decide to do so. But on a solo training ride I don't want to have to worry about braking or having someone accidently bump my back wheel.