A lot depends on your marginal tax rate. Mine is very high. About 45%. When I ran the numbers, I discovered that if I had to choose between accidentally undercontributing by $1 to my 529s and overcontributing by $2 and paying a penalty, I was better off with the penalty. (That's not to say that it's better to overcontribute than exactly-contribute --it's not. But you don't know how much you'll need, so you it's wise to figure out how to lead your target.) If your rate is high, then there's huge value on reducing your taxable asset surface.
Then I went into looking into ways to use that $2 of overcontribution. Turns out, you can point the money at yourself and enroll in a community college. Then you can use the money to pay your own living expenses, up to what the CC says is allowed for financial aid purposes. I'm pretty sure my wife could do this as well. I suspect, there's at least $50k or so of space in my 529 plans that I can blur into overlapping with my tax-advantaged retirement space, because retirement and education both involve living.
There might be other tricks you can use. Review the IRS pubs yourself, but as I read them, it looks like they only care if a qualified education expense existed, not what the money was used for. So if you have nephews in college without 529s of their own, there's no good reason why they or their parents should object to you pointing some of that money at them and withdrawing it tax and penalty free.
Finally, I think that pointing any surplus money at grandkids is the best kind of legacy you can leave. People are living so long that inheritances arrive too late to change anyone's lives. If you want to do something for your kids and grandkids with your money, best to do it while you're alive (if you plan on living long), and best to take advantage of any tax incentives Uncle Sam is prepared to offer.
Net-net: when I realized how much I might actually benefit from having surplus money in a 529, I changed my balance targets. Instead of shooting for saving for 4 years of public schools assuming in-state tuition and optimistic tuition inflation assumptions, I'm basing my targets on something closer to private school tuition. If that's too much, then great: on to a very good spectrum of "Plan B" options. If it's just right or too little, then I'll be glad as hell I was more aggressive!
I do recommend checking out that gocurrycracker link. The analysis is excellent.
P.S. Note on using a Roth for college: this really screws with financial aid. It produces income on next year's FAFSA form. The way the aid calculations work, it's better to have assets in a 529 than income from a Roth. 529 withdrawals are not counted as income for financial aid purposes.