The latest recommendation is to do a fecal egg count (FEC) before worming to make sure you're using the correct type of deworming product to target the types of worms your horse has. It's very quick and inexpensive to do an FEC- you can usually take a sample in to your vet's office- no need for a farm call.
Tapeworms and encysted strongyles won't show up on a FEC, though, so you should also deworm with a product that kills them at least once a year. In addition to the ones deserthorse mentioned, Quest Plus also kills tapeworms. Only Quest, Quest Plus and Panacur PowerPac will kill encysted strongyles.
This year, I've done two FEC's on my horse and both came up clean, so he only got Ivermectin in the spring (along with all the horses at my barn) and will get Quest Plus after the first frost.
Actually, Panacur Powerpac is the only one who kills ALL stages of encysted larvae.