If your PC is a desktop (not a tower) and it has vents on top (in addition to rear), then the hot air is going to rise from the top vents. Now if u place your LCD right on top of that, then that wouldn't be advisable.
Also if the PC has side vents, and u put your hands there and u clearly feel warm air, then placing a LCD on top of that (width of LCD sticks out beyond width of PC) then that wouldn't be good either.
Why not just place the PC under the desk as suggested? That way too, your ears would be farther away from noisy fans.
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