I often use my PlayBook in public hot spots (like Starbucks)
Generally, how safe is it to leave wifi file sharing on? My file sharing password is pretty strong...
Or should I just turn it off whenever I go into a public space?
To save battery life you could disable WiFi when not connected to a WiFi access point. Otherwise, with a strong password your tablet should remain safe from most crackers. You can always disable file sharing when out and about.
It should be safe since you have a password for file sharing enabled. I'd be more concerned about having web data captured over an open wifi than I would be about someone hacking into my actual device.
I suggest also to leave file sharing always on, when something happens to your PlayBook broken a screen or so, you will at least have an extra way of backing up your files.