I used 6% but readjusted my numbers annually based on the College Board report on college costs that comes out every October/November. The last five or ten years, 6% turned out to be too high for the costs I was including (TRBB&F at a public 4 year school) because my projections / planning just got rosier and rosier without me doing much.
Who knows what the future holds though.
Very true on not knowing what the future holds. I'd like to be overly prepared as the one thing that set my husband and I back for awhile was the student loan debt we graduated with. I see coworkers who had everything paid for them and wish I got to have that $0 start too. Competition and cost consciousness might make the 8% inflation expectation I had not nearly as bad, but 18 years makes a lot of time for inflation to pump up the number a lot.
Indeed, 18 years is also a lot time for the higher education system to collapse under its own weight. If typical college becomes say, $300,000+ per kid in today's dollars, it stops being feasible for most families to send their kids too and it forces change. I hear you on student loan debt, but having >$750,000 locked up in a 529 is astounding to me. The "risk" becomes they get scholarships or don't take a usual approach, or the fundamental nature of higher ed changes and that money is now not accessible.
Hypocrisy check: I have three kids with $0 in 529. Now that I live in a state with a tax break, I'd like to contribute a small amount to 529s, but I'm cash flowing grad school and a home renovation now. I'm mostly going the Root of Good route with advance credits, community college, and scholarships. Still have 7 years to plan.