Chris,

How much twist is acceptable from a safety standpoint? Or how much
twist is acceptable in your project?

You should keep in mind that a board could have internal stresses. You
could have a board that sits completely flat on your TS before ripping
it only to have 3 twisted boards by the time your done.

I've had good success using a planer to flatten a board
(http://www.taunton.com/finewoodworking/pages/wvt095.asp). I used this
method to flatten a board in prep for resawing only to end up with two
twisted book-matched panels when I was done. Ho hum.

If you don't have a thickness planer I would go for the hand-planes as
people suggested.

-Miles