any rules of thumb for a "safe" highest ire setting on the camera?

100IRE zebras are always considered the "safe" point but...

some folks like to use 90IRE as an extra safety zone but ...

70 and 80 are considered for checking skin tones.

Generally, though the answer is 100.

(Personally I use 90 when I am going to edit my own material).

I use 100. If it's zebra striped at 100 it is definitely blown and I can see in the VF exactly what portion of the picture are out. At 90% or lower I don't known how much of the striped area is over.