So far, I feel that using custom WB after shooting with a grey card works very well for tungsten lighting, than it does for fluorescent lighting. No matter what I've tried, I always end up with a blue tint when I use customWB under fluorescent lighting. But surprisingly, autoWB seems to work well for fluorescent lighting.
I don't think the 450D can set Kelvin temp for manual WB, but since I carry my grey card set in the camera bag, I haven't encountered any "problems" except when I forget to set WB before shooting.