Smoothing and straightening can radically alter certain shapes. Make sure you have a backup or at least have several progressions of undo set in your preferences before you start applying a series of straighten or smooth commands.
Flash might add more anchor points to curves after Undo commands or to images imported from other applications. Make sure you optimize complex curves to keep the number of total segments down for better playback.
Curves in your final SWF movie might be slightly different from the ones you developed inside Flash. Even though you are working with cubic Bézier controls, the SWF is compiled with quadratic curves. Quadratics speed up playback but are less accurate than cubics.