[m1ckz]

mine are in a cold greenhouse.doing ok but getting a bit tall,i was told to trim them down with scissors,,,is this a good thing to do

[upert]

how warm is your greenhouse? the days are still short and they're searching for light.

[m1ckz]

its not heated at all,but its light in there

[Stewpot]

You do not cut Onion's down if the youg seedlings get to tall, but you can do this with Leeks.

[mumofstig]

quite a few websites recommend cutting the tops off if the plant hasn't shed the seed casing cos that stops them straightening up.
Haven't heard it advised after that stage, surely you'd just be removing the plants food factory 

[Salmo]

You do not cut Onion's down if the youg seedlings get to tall, but you can do this with Leeks.

Oh yes you can!

If the onions are leaning over and touching the soil or getting muddled with each other giving them a hair cut is the best idea. Snip each one individually and clear away the bits.

Cut them at about 11/2 - 2 inches and they will then keep themselves upright. The 2nd and 3rd leaves will soon appear and take over.

Yes, in theory you reduce leaf area, but you will not notice any difference.