Valbazen is a Benzimidazole. Benzimidazole's are mostly excreted when given to poultry, only a small amount is absorbed into the bloodstream. There will be minute residue in eggs.
We eat the eggs after worming birds with Valbazen or Safeguard. I'm still here typing.
However, if you suspect that...
Since your soil is cold and wet, I recommend worming once every 3 or 4 months.
You're right. Our soil is warm and moist/wet most of the time and I worm monthly.
We're currently getting a nice break from the rain and we're enjoying it. :D