Yes. Most people who use herbs magically use them in dried form. Now, there is a limit - you wouldn't want to use very old dried plants that have lost most of their essence.
Plants aren't just a tool to focus your intent or to decorate your altar space - they have a magical ressonance all their own. Sometimes a good herbal mixture will do most of the work for you - no belief or extra work on your part needed.
Basically, i treat my magical herbs the same way as my cooking herbs. If they don't smell, if they don't have "life," energy, charisma, i put them in the compost pile. Depending on the herb, whether it's a dried root, flower, or leaf, you usually have a year or two before they start to go downhill - and of course you have to store them in the all-important air-tight "dark, dry space."