To me, practicing does not equal getting advice.
The matter is, when I come to a practica, I already got homework from my private lesson, something I am planning to work on. Therefore, all I need is music, sometimes people around, and a partner who can lead (if I practice as a follower). Since I mostly work on a very basic stuff, the leader does not have to be "advanced" or know fancy steps, just a good lead is more than enough. Of course, he/she is most welcome to try whatever he/she wants, new steps or whatever. It is understood that it may not work, or not feel good at first, and it is OK, because we are in a practica. We may talk to figure out how to make it work better, but to me, most of the time it does not require words. We are communicating with our bodies.
About getting advice at a practica. In my opinion, very few people are qualified to give valuable advice. I do not consider myself one of them, either. So, if somebody asks me for feedback, I try to keep it as simple as possible, and mostly positive, telling my partner what he or she is doing well rather than pointing out bad things (of course, if it hurts, I will tell).
Even when I had a steady partner, with whom we knew each other very well, were comfortable to work together, we hardly ever said things to each other in a practice session. Most of the time we assumed that each one of us was aware of one's "mistakes", and was working on correcting them.
I felt like the best feedback I could give to my partner was to follow his lead as honestly as possible, and he was in full agreement with that.
So, to random people, whom I might not know very well (at what point they are in their tango? what they are ready for? what do they expect from the practice session? they might not even be in a practica to practice, but to dance and have fun, and I guess it is OK, too ) I would not think of giving unsolisited advice.
Also, practicas are good for trying out new partners. Some people, I would not dance (at least for the first time) with in a milonga, but certainly in a practica.