Nuks, you don't mention what kind of screen.
If it's an LCD/flat panel, then don't worry about it. Either way, it won't make much of a difference. I'd say set it to 5 minutes, just so you don't have to worry about the screen turning off while you're in the middle of reading a long article or something. The power used by an LCD for those occasional 4 extra minutes isn't going to amount to much.
If it's a CRT, then it's a little tougher. CRTs do tend to have the most 'stress' while warming up, so having them turn off and on more often can cause more wear, shortening their life. But unless you're having it go on/off a hundred or more times per day, and are concerned about the long (years) term effects, I also wouldn't worry about it too much, and would set the timer for 5 minutes, for the same 'ease of use' reasons above. Even a 32" TV doesn't exactly use kilowatts of power. (I've measured my eMac, I've calculated that the CRT uses about 80 Watts. Leaving an incandescent lightbulb on is using more power.)
In my house, I leave 6 computers turned on 24/7 (doing distributed computing when not actively in use,) including some power-hungry Pentium 4s. Yet my 1500 Watt space heater uses twice as much energy when it is running than every other device in my house combined.