I use Westgate labs, very quick and helpful. Plus you can get reminders sent to you when counts are due (which I find useful). Also, because they don't sell wormers there is no bias in the information they give you.
The above posters are correct, worm counts very useful but in winter horses will still need to either be blood tested for tapeworm and wormed if positive, or wormed for it anyway, plus they must be wormed for encysted stage of small redworm. Both are potentially very dangerous for horses and neither reliably show on worm counts. Equest plus Equitape or as a single dose for a healthy horse, Equest Pramox would cover these worms.
Other times of year worm counts are the way to go.
You can worm count at this time of year too, provided it isnt instead of the essential worming above.
I use Westgate labs too, very efficient and helpful and I get a 4 test pack with a calendar.