Keying a green screen in a log format or after color correcting/grading? What is best?

My workflow depends entirely on what I am expected to deliver. I do a lot of projects where the folks doing the final color grade only what the matte, and they want it in the same color space as the camera original. The last keying/composite project I worked on came with a trimmed copy of the original footage, the LUT the editor used for the edit, and the background plate as camera original LOG footage. I added an adjustment layer above the background plate and applied the supplied LUT to the camera original. When the composite was complete and the color reasonably well-matched, I turned off the background plate, added the original footage without any color correction, and used the color-corrected and keyed footage as a track matte to give the editor a copy of the original footage with a hole punched in it. 

I also used the Keyed footage as a track matte for the background plate with a few additional adjustments to give the editor an uncorrected background plate just for a light wrap layer. As a starting point, I also supplied my color grading LUT for the background plate. This project also required some hand roto, Rotobrush, and cloning to get a perfect matte.

In the end, I delivered my whole project file, a rendered copy of the original with no color correction with a hole in it for the background, a light wrap only layer that was also without color correction, The background plate with transparency (also not color graded) and a color graded composite. The colorist then could efficiently run color grading on every part of the composite using my LUTs for a starting point. This workflow eliminates about 90% of the masking that the colorist would need if I had just supplied a color-corrected composite.

The keying composite I did just before that job was delivered as a fully color-corrected 32-bit image sequence that I matched to the other shots in the sequence. The 32-bit file still gave them a lot of room to work on the final color grade. That is what the client wanted.

Long story short: Talk with the client and supply what the client will need to complete the project in the most efficient way possible. No matter what you do, you will have to add some color correction to the original LOG footage before you can pull a good key. You may send up using the key as a track matte for the original footage or render a color corrected copy they can drop in the timeline.