I'm not sure where you read this, but I don't agree. If you have the time and possibly a flash meter, setting the flash(s) to manual and your camera to manual may give you more control and more accurate exposure, but TTL is hard to beat for quick shooting situations.
Note that TTL and TTL-BL are 2 different settings.
The BL in TTL-BL is for balanced. Your camera will try to balance the ambient light with the light produced by the flash. You should use this mode when the flash is used for fill. For example an outdoor portrait with the subject's back to the sun. Often in this mode you will want to dial in some negative flash compensation to keep the fill looking natural.
You should use TTL when the flash is the primary source of light, such as photographing indoors at night. Your camera will ignore the ambient light and set the exposure only based on the flash. The background which is lit mostly by the dim ambient light will probably be dark in your image.
Gary in SE Michigan, USA. Co-organizer of the Southern Michigan Chapter Nikonians membership - My most important photographic investment, after the camera. D4, D810, D300 (720nm IR conversion), D90, F6, FM3a (black), FM2n (chrome) YashicaMat 124, Graflex Speed Graphic 4x5 My Nikonians Gallery & Our Chapter Gallery