It depends on which version of the program you're using.
Version of Premiere Elements prior to version 11 don't mix video formats well at all!
It also depends on the specs of your video. If you are trying to mix NTSC (~29 fps) and PAL (25 fps) -- well, that's a pretty tough mix, and I wouldn't recommend it.
It's Elements 11 but, yes, I take your point.
I'll avoid it!
Thanks for pointing me in the right direction!