it all really depends how you use them. anything "semi-aquatic" must be planted so that most or all of the leaves are outside of the water (draceneas, lilies, and pothos fall in this category) but can still be a nice addition to your tank. if you try to plant them fully suberged, they will die and rot over time, which will poison your water. other than that, i don't think i've every heard of any plants being patently "bad" for bettas.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)