I've run 25.4 in 26.0 and 26.0 in 26.4 with no real problems, both times shimmed with a Coke can. Never died.
Edit: It also depends on the bars you want to run. I'm pretty sure you'll be hard pressed to find 26.0 riser bars, as opposed to 25.4 or 31.8, but (most) of the older Cinelli bars are 26.0 or 26.4. Nitto drops are 25.4 for their NJS track drops and other bars, but they also have some of their other models in 26.0.
I've run 25.4 in 26.0 and 26.0 in 26.4 with no real problems, both times shimmed with a Coke can. Never died.
Edit: It also depends on the bars you want to run. I'm pretty sure you'll be hard pressed to find 26.0 riser bars, as opposed to 25.4 or 31.8, but (most) of the older Cinelli bars are 26.0 or 26.4. Nitto drops are 25.4 for their NJS track drops and other bars, but they also have some of their other models in 26.0.
Confused?