I worm regularly and have had no issues with it. I use Fenbendazole (Panacur/Safeguard) in the water most times, but twice a year I hit everybody with a dose of Eprinex between the shoulder blades for internal and external parasites. If you have turkeys and chickens, deworming is really important. Histoplamosis causes fatal BlackHead in turkeys. It is a protozoa that lives in the liver fluke (worm) of the turkey, and is carried by chickens, who don't seem to be troubled by it. Deworming with fenbendazole kills the liver fluke and removes the 'home' of the histo in turkeys. Histo is common in this area, but in all my years of raising turkeys I have never had one affected by it. I like to think it's because I keep them dewormed.
Most people think Wazine is a good poultry dewormer, but it's really not. It doesn't get many species of internal parasites in poultry. I have also tried pumpkin seeds and DE and have not had any luck using those as dewormers. I don't think DE kills external parasites either...and it can be fatal in ducks.