It's a good question.

On a recent ride (just a 50 miler) I worked like a dog to catch up with the wheel of a very experienced cyclist, one with whom I've ridden many a time and trust completely.

When I wheelsuck I often get within six inches of the wheel. This made the guy up ahead nervous, as if I trusted him not to stop abruptly but I'm not sure he trusted me not to rear-end him.

So I keep that in mind. I think it would be polite to say something and ask if someone would like to work together.

On one tour I did ask and the two riders up ahead of me made it clear that they would prefer to ride on their own.

Having said that, I would consider it a great compliment for someone to wheelsuck me. Why, I'm so slow a glacier wouldn't even bother to draft on me.

Addendum: it doesn't count if the cycle up ahead of you is a tandem. If you own a pick-up truck, people will ask you to help them move stuff. If you ride a tandem, people will suck your wheel. It's just the way it is.