Anthony is right. Slow sync is intended to be used in low light conditions to allow more ambient light than normal sync would provide. During bright conditions normal sync and TTL-BL should work well. Note that you can also include some flash exposure compensation to either increase or decrease the amount of fill that the camera would normally provide.
There is one more case to consider if you have an SB600, SB700, SB800 or SB900 external flash (and probably a couple of other recent models): If you want to use large apertures in bright light (and, consequently, high shutter speeds) to limit the depth of field you might want to use Auto FP High Speed sync which will allow you to use the flash at shutter speeds much higher than the 1/250 normal maximum flash sync speed.