Do plants grow in rivers and creeks etc. that are exposed to sunlight?
Sunlight contains UV-A, UV-B and UV-C does it not? (UV-C being the main type of UV light a steriliser emmits)
Therefore i can't see UV sterilisers doing anything detrimental to your plant tank. Besides, UV's are only for altering DNA molecules in cells of living organisms. A UV is not a carbon filter, nor an RO unit, so it wont alter the chemical composition of your water (which i assume is what your worried about).
Hope this helps,
Aaron.
PS: I have run UV on a planted discus tank for a long time with no ill effect.