By default, important data would be in ~/.local/share/bitcoin. That's really helpful because it means that simply backuping .local is enough (and you don't have to care especially about bitcoin)
It is advisable to backup the wallet.dat, yes, but using ~/.local... adds the problem of proliferation of copies of your wallet.dat, increasing the risks of a copy of it becoming available to a thief. Currently, the wallet.dat file is not encrypted while in use, so backing up the wallet.dat file anywhere is akin to leaving your checkbook on the counter at Wal-Mart while you go out to your car.
As far as I'm concerned, I use a ~/Private encrypted virtual directory (using ecryptfs). My wallet.dat in ~/.bitcoin is thus a symlink to ~/Private/wallet.dat.
I would not feel comfortable adding an other level of symbolic link, so I won't use ~/.local/share for bitcoin.
But I guess if someone feels adventurous, he could try that.
