If the flash units are close to the group, the distance between members sees a wider light fall off. So to get even lighting you might want to figure the fall off related to the Inverse Square Law. Further away from the source means less light but less difference in front to back rows or from nearest member to furthest. A flash 10 feet away will be a lot of light drop off to the rear row that is 12-13 feet way. A light source 20 feet away will see a lot less drop between rows. This usually requires strobe with defusion if you get much further than 20 feet. Bounce can work sometimes but there is even less light reaching the subjects because the Inverse Square Law again comes into play by attenuating light equal to the distance to the ceiling plus back to the subjects plus a reduction factor of the absorption of the ceiling.
If there is ambient light to help you be sure to gel your flash units so they match the color temperature of the ambient or you will never get the colors right.
It helps to get the lights up so there is no shadow on the background from the camera's point of view or move the group father from a wall so the fall of is enough to keep the back low in illumination.
If you have access to the venue before the day of the shoot, go experiment so you have decided how to light it before the choir comes. You can use a couple coat racks, one closer to the camera and one representing the furthest row. Try bounce to the rear wall to avoid the raccoon eye look of dark eye sockets which is the result from light source coming from too high an angle. The coat rack or even chairs will give you a reasonable idea of shadows, relative light levels front and back row and color temperature conflicts with ambient and the flash. This will make the real session much smoother and professional since no experimenting is needed when the people are waiting or getting frustrated by delays.
Good luck