The worm eggs are all over the ground regardless of whether you place some poo in your garden or not...just from the chickens walking the ground. So I'd not worry too much there about it...the one thing I have read is that for any poo of any animal- you don't want that applied to the land immediately before planting due to risk of bacteria getting on your produce- one place mentioned 120 days that I saw on internet.
So if you turn your poo in a few months before you till and plant then your veggies should be safer. Exposure to sunlight of the soil for one year or more is what I have read in the past, for what it takes to kill the worm eggs from roundworms. The following two websites give a time frame:
http://msucares.com/poultry/diseases/disparas.htm
http://poultrykeeper.com/general-chickens/worming-chickens
I don't know if using black or clear plastic over the garden area first would help to cook the surface soil after tilling in terms of worm infectiveness, but remember that the area around their coop will be covered in worm eggs too, so personally I wouldn't do it, but I am no expert.
http://www.ehow.com/how_8216486_use-black-plastic-sterilize-soil.html
You may want to do the Balfour method (have two garden areas and put the chickens in the other area not being gardened, then switch so the tilling and cleaning of weeds has been done by the chickens. Puts them on new soil each year.
Personally, I have noticed that once having had worms on my soil I now must deworm twice a year (and repeat at 10 days). I see the worms in the poo- there is no question as to whether I definitely have a problem or not. So that is my response and I just assume the ground is covered in worm eggs...sunlight will hopefully kill them off and I treat my chickens to save them at the time. I repeat as needed.
One buddy tells me that he knows people who have to worm every three months, being on nasty wet soil. Remember to toss eggs (I toss them for at least two weeks after a dose) as these wormers are off label for egglayers- there are no approved wormers for egglayers. If you sell meat or eggs then wormers may have an effect on your ability to sell meat or eggs. I don't know much about that.