Do we have a different vision on the back problems?
From what I feel, many people are suffering from lack of power in the back. In such a case paschimotasana should be done with care, and correct way – without overstretching lower back. If it is done wrong way – spine becomes like a loose necklace. And constant pains and slipped disks just around the corner.
I say paschimotasana yes, but sarpa, setubandha, salamba – first, then carefully add bhuja, dharuna and - balasana – must do at the end and may be in between. It is very important to get right balance of Ha and Tha there.
To finalize is all: general question – general answer. Don’t hesitate to talk to yoga tutor before or after class. Serious problem – personal consultation. If the class does not work – drop it. Pay for personal lessons with very good yoga tutor, just ask people for one. Or go to the hospital.