Is he inside at night?
How cold does it get where you are?
Does he have shelter in turnout?
I body clip my dressage horse every year. I leave his belly and legs because we can get down to a balmy -40F for a few weeks every year and horsey gets upset if he does not get his outside time! He comes in at night when it's cold, but there is also adequate shelter outside. What I use for him:
Cotton sheet, 250-300g stable blanket, 250g turnout blanket no neck, 350g turnout blanket with neck.
How I blanket him when he is clipped:
15 to 35f - cotton sheet and 250g turnout blanket
0 to 15f - cotton sheet and 350g turnout with neck
-30 to 0f - stable sheet (or Back on Track) and 350g turnout with neck
below -30f - depends on windchill but usually just his BOT, stable sheet and neck blanket are OK for the cold snaps, if it gets super cold I'll put both turnout blankets on him.
For the outside boarded horse who will be getting a modified trace clip this year, I'll be using similar blankets except he has no BOT blanket (poor kid) and will not be coming in the barn when it's colder than -25f. He has adequate shelter with lots of straw outside too. Of course when it's really cold they should also be getting tons of hay!
Good luck! And as far as for shows - no one really cares as long as the clip is done well. If it's some raggedy job then that's not so good, but a well done trace or modified trace is definitely acceptable at shows.
As far as when to clip - whenever drying the horse out becomes a nuisance. I usually leave it until late November so I don't have to re-clip but if you're going to clip multiple times clipping now is not an issue. Just don't clip after winter solstice (Dec 22nd I believe) unless you are doing a full body clip or your horse's summer coat will be affected.
They say money doesn't buy happiness -- well happiness doesn't buy horses!