It depends:

“As long as you have the right 8-bit, youre fine." DSLRs like the 5D Mark III have 14-bit sensors, but for video capture, they’re converted down to 8-bit. Its this 8-bit “secret sauce” that the experienced engineers are able to pull off.  “If the engineers are getting the right 8-bit in that final codec, its visibly lossless,”

via Misinformation: The Right 8 Bits | HDVideoPro.com.

It also depends on the lighting in the scene. If the scene has a high dynamic range, more bits are better. If the scene’s dynamic range is shallow, fewer bits are needed to capture the full range. And, like the above, it also depends on how the codec itself works to capture in 14-bits but mix down to 8-bits.