Bitcoind live wallet backup (by copying wallet.dat) Is it safe to copy wallet.dat while the daemon is running, or should the daemon be stopped before doing so?

I'd imagine it would be safe unless addresses are being generated exactly at the same time.
In some cases, bitcoin-qt/bitcoind may regularly (and even gratuitously) flush the wallet [1], so it's possible that wallet.dat could be in an inconsistent state at the time you make your own copy.

The backupwallet RPC call should be a safe way to accomplish this.

[1] https://github.com/bitcoin/bitcoin/issues/10236