Is it all right to leave a toaster plugged in all the time?

We're sitting at my grandparents kitchen table, the elecric toaster plugged in, but not in use (and had not been in use yet that day), when all of a sudden, gray smoke starts drifting out and the curtains above the toaster burst into flames. I hate to think what would have happened if we hadn't been sitting right there to act promptly.