OP (and Heath a little)
I'm on a mac and I use MPEG streamclip to convert all of my files to ProRes 422 LT before editing.
Evan was right when he said that ProRes doesn't make any difference for grading, BUT I have noticed from experience that Premiere (and any other software) can handle ProRes much more efficiently. When I just use H.264 from the camera, sometimes it lags, I have to render more, etc., but with ProRes, it works flawlessly every time, no rendering.
Another thing I like about MPEG Streamclip is that I can trim my clips. When I'm going through all of my shots in one day, I drag one into MPEG, click Batch List, set in and out points, then click export. Then I name the file as per what kind of content it has (My way of naming is: trickname1-cameraangle), so that when I've converted all of my files to ProRes, I don't have to sort through them when editing and can just find a file by trick name.