I have always left mine on standby the reason I ask is recently I had a new virgin tivo box installed and the v box moved to another room and the v box was making a sort of humming noise as though it was continualy recording I phoned virgin to ask if this was normal and I although they seemed to have fixed this over the phone she did say to me it was safer to turn them off at the mains when not in use.
My TV would cost 9p to leave on standby for an entire month (730hrs). If you're worried enough to want to save 9p a month then I'd suggest there are other places to look that would save you much more.
No, it is not unsafe but very very wasteful. Did you know that the amount of electric power wasted in the U.K. leaving things on standby and charging all the time amount to about £750 million pounds? This is in comparision to the amount (2%) lost in transmission and transformers which is about £500M.
I hope this brings it home to you all. Do NOT leave things on all night, switch them off when not needed.
The tiny amount you save is not worth mentioning. As for safety, modern appliances may have have inbuilt safety to leave on standby but there is always a minuscule chance that something may be amiss and a spark could (and has) start a fire.
You can make up your own mind on the links below. The first one mentions that "standby uses 80 per cent of a TV’s power" which makes me suspect on how much they know about electronic appliances.
WOW, how'd they come to that figure, my TV uses about 250W when on and 0.5W when off, I make that about 0.2%
(though while looking at figures for this question I did discover the Xbox360 uses a hell of a lot of power while on standby, I might start unplugging mine when not in use from now!)
They can catch fire, as can any other electrical appliance. switch off if you can, but we go away for a few days each week and have to leave the fridge and freezer on, and i always worry that When we return the house has burnt down.