It depends on what you're feeding them. If they get the recommended amounts of concentrates (fortified feed or grain) for their age and weight and the hay you're feeding is high quality, then she shouldn't need any supplementation.
If you're feeding less than the recommended amount of concentrates and/or your hay isn't top quality (fertilized and tested), then you might need a general vitamin supplement.
Horses that are stalled should get probiotics and a calcium supplement (or alfalfa pellets) to keep the gut healthy and prevent ulcers. Free choice grass hay while stalled is a good idea for that too, and to keep stall boredom to a minimum. Horses that travel a lot should also get the same to prevent stress ulcers.
My horses are pleasure horses (moderate work, little to no travel). They get a diet of high quality Bermuda grass hay (25-30 lbs a day), 1.5 lbs of Alfalfa pellets, a general vitamin supplement, and 1 cup of a custom blend from horsetech.com: Nutra Flax w/BioFlax mixed in.
The Flax/Biotin combo provides healthy fat, keeps their feet healthy in our muddy pasture, and keeps a nice sheen to their coats even in the winter.
The general supplement is to supplement for selenium, which our soil is low in, and a "just in case" to cover all the bases (I take multi-vitamins daily too). Right now I'm feeding GrandVite. I've also had good luck with Balance II, Select II, and Smart Pak's SmartVite Maintenence Grass.