The bond index funds tracking the BBgBarc US Agg Bond TR USD hold about 70% US government debt including treasuries, and about 30% investment grade corporate debt. They generally behave quite well during stock downturns. So good enough for me.
Yep - it's the most common choice and a perfectly fine one indeed! As I mentioned above, bonds can behave well during downturns, especially the last two big ones. That hasn't always been the case, so I tend to think of them more as just another uncorrelated asset. Even with that said, treasuries tended to do even better during those 2 downturns, but I'm not counting on it. For me, it's just a way of completely separating out my bonds from the corporate world, since I already have plenty of coverage via my stocks.