Using manual flash for back lighting is fine as long as you know what you are getting into. Honestly for studio work, you want to be in manual for all your flashes as your shot to shot consistency is much higher in Manual mode.
I set my Off Camera Flashes to ETTL and then control them from the camera menu as Manual mode (in two groups, the 7D has 3 groups I think).
This way I can have the control of the Manual mode in studio shots, while not having to scramble around and re-set the OCFs up on stands and into the umbrellas etc.

