I've converted videos at different frame rates without any significant impact to the playback or quality. My guess is that if you output 23.976 fps to 24 fps, it is unlikely that you (or broader audience) would notice any issues. I would suggest not having Frame Blending on. When you increase the frame rate, frames will be added and frame blended makes these new frames a blend of the frame before and after (so a semitransparent overlay). This can work in some situations but not for others. If there is not a lot of movement in the video, adding frames without blending should look fine. The best way to be sure is to test a different approaches and see what looks best to you.
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