I guess it depends what you're using. I tried expanding one with a live CD Gparted once and it was a disaster. I had started using it though, thinking it had already been resized. I usually don't know the right block size to use with dd so I use Win32DiskImager to write images, and I have no partition table editor for Windows.
I suppose it could be unmounted safely enough but you should change the partition table before you start using it? Then again with splitfs that doesn't matter.