I don't believe I have ever enjoyed Crescent Lunge. I think my lumbar spine is too arched, I never feel like my hips are on the same page, and I always sense that I'm one second away from toppling over. I've played with this posture for years to find some sense of ease. Even with all the experimentation I still don't feel comfortable while holding the shape. I've just come to terms with the idea that I may never feel great in the posture and that's okay.
For the majority of my practice and teaching I emphasized the drive of the back heel toward the back of the mat. The energetic press of the heel back gives the calf a great stretch and helps me feel more stable in a fairly unstable posture. Last year I was taking a class in LA from a teacher who was new to me. During practice the teacher came around and pushed my heel up in Crescent Lunge. Full disclosure, before the adjustment even happened I was instantly turned off by some statements made by this teacher in the first couple minutes of class. I instantly established a terrible attitude toward this teacher and let that attitude bleed into how I received her adjustment. "I do this posture this way and I don't like how this teacher wants me to do it."
Fast forward to this year, my teacher expressed his experimentation with lifting the back heel high in Crescent Lunge. Wait a second... no... Ugh. I respect his knowledge and perspective so I gave it a shot. Okay. The LA teacher and my teacher have a point. (Side note: The comparison between the LA teacher and my teacher has been a huge learning process for me. I'm a self described yoga snob and I need to constantly check myself in practice with teachers I don't know well. All teachers have something to teach me and I need to respect that process.)
For the last few months I've encouraged my students to lift their back heel over their toes in Crescent Lunge. For many it makes the posture less stable, however, it forces more muscles in the legs to fire which in turn creates stability. I have a lot of highly mobile students in my classes and the lift of the back heel forces them to engage the legs and glutes and not dump into their flexibility. Also, since so many students, myself included, have been programmed to drive the back heel back, a tiny shift of alignment forces them to stay forced and mentally engaged.
I'm not saying one is right and one is wrong. I'm saying it's worth experimenting. Try out the two variations. Does one feel better for you? What differences do you feel between the two options? Yoga is all about exploration. Give yourself permission to explore and go from there.