Many Argentine Tango teachers start beginners off with the so-called 8 count basic - the leader steps back, side, forward to the cross, then a forward-side resolution. Most of my students know how I feel about that.
The 8 count basic starts with two extremely disruptive, often socially unacceptable moves - a back step against the line of dance, followed by a side step toward the center of the room. Neither step is intrinsically wrong. But both can be very wrong in certain contexts.
Consider that side step. When dancing Tango, I often think in terms of a traffic lane. When you are dancing Tango in a large open uncrowded space, your lane is quite wide, and you can comfortably step to the side without leaving your lane. However, on a very crowded floor the lanes are very narrow. The only way to take a side step without leaving your lane is to take it along the line of dance.
It is like the difference between moving into the left lane on a divided highway, and moving into the left lane on a crowded 2 lane city street. The first is both common and exceptable. The second will get you a ticket.
Ditto back steps. Example: if you are driving down a deserted country road and you miss your turn there is no problem with stopping, backing up, and making your turn. But if you did it on a crowded city street you would get a ticket. Same with back steps in Tango. The more crowded the floor the less acceptable they are.
Side and back steps are a part of tango. Everyone uses them. If the floor is crowded we use them carefully, sparingly, with great awareness for those around us, and keep them very small. If the floor is packed like a subway car at rush hour we cannot safely use them at all.
There are no Tango police handing out traffic tickets for dangerous navigation at a milonga, thank goodness. So we must police ourselves, dancing with courtesy, care, and consideration.