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The "" (empty-string-named) account is allowed to have a negative balance. You can sendtoaddress as long as the entire wallet has enough coins.
Accounts (like labels before them) are just a useful accounting mechanism. The rest of the network doesn't know or care what accounts you have. And although transactions to and from the wallet are credited or debited to accounts, all of the 'coins' get mixed together in the wallet, there is no notion of "this account received 100 bitcoins in this transaction, so we should use those for that transaction out..."
For example:
100 bitcoins are sent to an address associated with Account A. A's balance is now +100.
50 bitcoins are sent to an address associated with B. B's balance is now +50.
100 are moved from A to B. A has zero, B has 150.
B is allowed to send 150, but it won't necessarily be the 100 originally sent to A and the 50 sent to B; if other accounts have received coins (transactions), those might be sent instead.
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bitcoin doesn't care (use any valid JSON string for the name), and the rest of the network doesn't care, so use account names that make sense for your application.