Well, here's the thing. Thing being my opinion...
24 frames a sec isn't a ton less than 29.97 frames a sec.
That said, I prefer working in Shake in "p" (progressive). If you do "i" (interlaced) you need to set your "Inc" (lower right corner of your workspace) to .5. This will allow you to step thru on a "field by field" basis. Which seems to take longer and make work a bit more difficult.
Also remember that Shake does a pretty darn good job of deinterlacing. You'll find that under "Timing" I think. And a lot of Shake's cool stuff (like camera stabilize) only work if the footage is deinterlaced.
Have fun with the retiming/optical flow stuff. It's AWESOME.
Pat Ortman
Empty Street Films