I also think it is better be overdressed than underdressed. I once wore a suit to a place whose dresscode was business casual, and they definitely did not expect that, but they liked that I did go above and beyond with what I wore to the interview because they said it showed I cared more. This was for a non-pt aide job though.
I went on a job interview for PT aide, and for that, I wore business casual. I wore a nice top and dress pants and some nice flats, but honestly I have no idea if they thought I was underdressed or not. When in doubt, I would overdress rather than underdress.