I agree elbows slightly forward is acceptable. I also agree that elbows behind the spine is not. And I also agree that an asymmetry with the right elbow further forward is acceptable. However, I think that perfectly flat across is also acceptable, though it can be more difficult to maintain.
If you’re going for the straight elbow line, as for example the Hilliers did, an exercise to get there is to first hold both arms straight out to the side, then bend the elbow joint only to make your desired contact.
That also depends on individual dancers in the couple (length of the arms, depth of the trunk, height difference ...) and of course, frame is slightly "breathing" through the dance (depending also OP, PP ...) - talking about int. standard. Checked that JnJ pro ballroom competition that I posted in other threads. On average, men had both elbows slightly in front of plane of the back with almost no asymetry, while ladies had left elbow slightly more in front of that plane than the right (tango especially due to different hold). Ok, here and there some elbows slipped behind that plane for a moment ... Those couples did quite a lot of shaping which wouldn't be possible with men having elbows in the back plane like it was in Hillier's time about 40 years ago ...