I know it ain't a bus, but I had a tire blow out on my Suburban. The tire was the spare that was put on due to road debris killing the new pirelli. I put on the old firestone spare & aired it up to proper spec. no checks or cracks- it looked almost new. The firestone was the correct tire & matched the others in size & weight capacity. I checked all tires at every stop. Less than 15 minutes after a rest stop, it blew out at 60+ mph. (not fun in heavy traffic pulling a 32' camper!)
I used to believe that you could judge a tire by it's looks. I too scoffed at those who insisted they had good looking tires fail at 6 years old.
NOT ANY MORE! That blowout convinced me that it ain't worth the risk.
If my max speeds are low (55 mph max), I might reconsider if the tires are only lightly loaded (less than 50% of rated capacity) & dualled out, but I'm definitely not taking chances on the steers. . . .
Be safe in your travels!
edit to add:
Rubber is not water proof, which means moisture can migrate thru the sidewall to the steel belts & once they begin to rust, a tire failure is on the way.