Let's list the reasons for making an inflation assumption:
1) I have a non-COLA'd pension
I think the important assumption is the difference between investment returns and inflation. Historically, that has varied a lot.
In a planning worksheet, I'd rather assume 0% inflation, then enter my investment return as a "real" rate. If I had that pension, I would make an assumption about how fast it shrinks. Then my numbers make intuitive sense.
Note that if I own any TIPS, I've got a locked in differential (vs. the CPI). The 20 year yield today is about 1%.