Keeping TIMELINE TIMECODE visible (not clip t/c) when exporting as QT file

Newer versions of QuickTime Player can display timecode in the small window at bottom left, where the hours/minutes/seconds indicator is. Click on it and you can choose between Time, Frames and Timecode.

If you want it to be a lot more obvious, you can place a timecode generator filter over your video in FCP. Go to Effects > Generators > Video and place the filter on a higher, empty video track using Superimpose (F12). It can be set to your Timeline's TC or any other value you might want to display. This is permanently "burned in" to your exported QuickTime file.

Several ways to do this, but what I prefer to do is export the timeline to a QT Movie, import that back into the project and drop it onto a new video track in the same timeline.

If you do this, be sure to make your corrections and changes beginning at the END of the timeline across ALL tracks and work backwards. Otherwise, your TC numbers will change as you make each correction, and your brain will hurt.