I looked at the assembly dump of a "large" project and the only sreg reference was from delaymicroseconds (ver 0011). So that corroborates what you are saying Don.
I think the only thing (generally speaking) we care about is the global interrupt flag in SREG, and being able to restore it to its previous value after ensuring interrupts are disabled for a critical operation.
I don't know if the general population needs to prepare for nested interrupts though.
One other annoyance is the fact that the interrupt handlers use reti by default also complicates things in the reuse department. If they pushed and popped sreg (you know what I mean
), that might be better from a reuse, nesting perspective.