Every 8 - 12 weeks between the start of March and the end of September 

NO!!! FEC's are used to look primarily for roundworms in equines. They do not cover all endoparasites. 

It is still recommended you deworm for encysted redworms and test for tapeworms twice a year, treating as necessary.

If you do not test for roundworms as well and just treat your horse, you could be selecting for resistance.

For example, a horse is treated with an anthelmintic that targets both tapeworms and roundworms (e.g. a double dose of Pyrantel) in early Spring. 

A saliva test for tapeworms is performed and the result is positive. The owner decides to treat the horse with an anthelmintic that targets both tapeworms and roundworms, rather than perform an FEC as well to check the roundworm burden, which is in fact low and does not require treatment at this time. This treatment promotes the roundworm population to develop resistance to the anthelmintic used (Pyrantel in this case), potentially making them more difficult to treat for roundworms in the future. Treating the horse with an anthelmintic that will target only the tapeworms (e.g. Praziquantel) in this instance would be more beneficial and would save you having to perform a second dosing.