Here are my somewhat rambling thoughts on the matter. I go to a weekday training ride where we ride in tight paceline formation. There are about 8-10 people in the "B" group who show up regularly enough and we have a certain level of trust among us. Every once in a while, however, someone new shows up and we're all on guard. I find myself getting more and more assertive in speaking up if that person--or even one of the regulars--is doing something squirrelly, like overlapping wheels. I hate to admit, however, that every once in a while I still do something kind of boneheaded myself. It really takes A LOT of practice, not just to draft, but to train yourself to react to situations--like a stray dog or road hazard--in a calm manner so that in trying to avert one problem, you don't cause another. It sounds easy until you're in the middle of it.
I do a lot of club rides on the weekends, which generally attract between 75 and 125 riders. At these, I often find myself either on someone's wheel or with someone on my wheel. There are so many people that it's almost inevitable--at least at the beginning before we all get spread out or as we leave a SAG. Especially for the first few miles of a club ride, I always assume someone is behind me and, therefore, use appropriate hand and voice signals just to be safe. I also sometimes find myself in a paceline at club rides with people I don't know very well, but I can typically get a sense of whether it's a safe or a squirrelly group pretty quickly. I now recognize the club regulars and generally have some idea of how safely they ride. In any event, if somebody starts to ride unpredictably, I back off or speed up.
However, as the ride wears on and we spread out, I'm very much less likely to attach myself to someone's wheel and am a little more weirded out when someone does that to me, especially if they don't make their presence known. And if I'm riding on my own, I never just draft off a stranger's wheel if I encounter them on the road and would be more than a little weirded out if someone did that to me. At most, I might have a short chat with them and then allow one or the other of us to go on ahead.
Beyond the safety issues--which are paramount--I think you have to have some appreciation for group dynamics to know if, when and how to approach other riders on the road. I did an out-of-town two-day event ride with a friend in May. During the course of the ride, I realized that we had very different approaches to the issue. To illustrate: As we were leaving a SAG, we found ourselves in the middle of a big group of people from the local cycling club that was sponsoring the ride. They quickly formed a loose paceline and my friend went to the front of the group and rode there for about 15 miles. In contrast, I stayed to the back of the group and more or less just minded my own business, kept my guard up and tried to just ride safely. Fortunately for both of us, they ended up being a rather friendly group. My friend chatted with several of the riders, while I got at least some sense that they knew what they were doing. Later, we hooked up with part of the group again at another SAG and road the last 20 miles with them. I didn't hang back as much this time, and they were more than willing to incorporate us into their group.
On day two of the ride, we road off and on in clusters of other riders because the route was crowded until after the first SAG. After we'd spread out, we came up on a group of about five (what appeared to be) friends riding together. My friend attached herself to the back of the group, while I hung back about 20 yards. From my vantage point, they didn't seem like they wanted any company. I, myself, didn't feel comfortable riding that closely to them, but if she noticed, she showed no signs of backing off. After a few miles of this, they finally sped up and left her.
My friend doesn't do any organized paceline riding and, in my opinion, has less appreciation for the risks and skills involved. As such, I think she's a little more aggressive/oblivous when it comes to drafting and the dynamics of group riding. I'm much more cautious and tend to hang back. I think it pays--both as a matter of safety and courtesy--to keep your distance from other riders until you can discern whether they're safe to ride with and/or want your company. I don't have one hard or fast rule that I live by in that regard other than to be observant, to ride defensively and predictably regardless of who's in front or behind me, to announce my presence and, if necessary, ask if it's okay.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher