Mixed Frame Rates Question

Hello

I've recently started some editing projects that will ultimately end up on DVDs and Blu Rays. These projects typically consist of interviews and film footage. Sometimes, the frame rates of these rushes differ. 

I've used Premiere for many years and have frequently used mixed frame rates in my timeline. Almost always, nothing ever looks odd or jerky - and, after doing some research online, I believe this is because Premiere conforms all clips to match the sequence frame rates?

Despite never noticing any errors, I have now had feedback that sometimes the film clips I use in these interviews are out of sync. However, everything seemed fine in the timeline and when I re-watched the exported file.

For example, I'm currently editing an interview that was filmed in 29.97. I've used that clip to set my sequence settings. I've dropped in some film clips that are encoded at 24fps. Everything looks and sounds fine. The exported file plays without a problem as well. Yet is it possible for these film clips to play out of sync later down the production line?

One workaround I've tried (in order to prevent issues later on) is to create a new 29.97 sequence and drop the entire 24fps film clip onto it. I then exported the film using the 29.97 frame rate. Checking the QuickTime movie inspector, the film does seem to be encoded at 29.97. Yet the exported 29.97. file is exactly the same length as the original 24fps one (and seems to play identically as well). Wouldn't there be a difference in length if the frame rate has been correctly changed?

I'm currently scratching my head about this so any help or advice would be humbly appreciated.