Grumpy_Mike:
Well they float high, I wouldn’t say it was safe because they are prone to EMI pickup in this mode.
Unconnected pin has quite low capability for EMI pickup. I made a very loosely related "experiment". In my project I had standalone ATMega with reset pin unconnected. No reset was caused in about half a year. After adding a programming connector (about 10 cm of wire) there was a reset about monthly.
I would guess a floating TTL input is HIGH at least 99.9% of time. Any LOW period will be very short. I can imagine plenty of situations where this is no issue (i.e. Enable of a 7 segment LED display driver). OTOH HCT input may float to LOW for considerable time period.
But you are right, "safe" was not the best word to use there.