Very much as Marigold has posted Icemaiden, depends on 3 things, information regarding their previous environment ie free-ranging (meaning exposure to potential infestation, dense poultry populations etc..) historical worming programme and also the weather. This time of year is a good time, but not really necessarily once the temperature cools and grass stops growing any worms then are in decline.
With all these points in mind above then factor in the birds age, I don't tend to worm youngsters until coming up to their first autumn (early hatch) or a late hatch bird (post July/Aug) it will be their first spring.
In addition you consider also that there is the option of testing worm burden through fecal worm counts, a few vets offer this for around £10 and also the chicken vet ( http://www.chickenvet.com) offer a postal service. All my birds are tested spring and autumn, their worm counts have been so low I actually have cut out worming recently at times as it was just not necessary.