I used to, then I stopped. I should note I've had a few communicative diseases transmitted to my display tank after I stopped quarantining. One griffisi angel gave my Chevron some disease that killed it within 48 hours. I'll be quarantining again for my new tank. I keep new fish in isolation for 2-3 weeks when possible.
Ideal qt would be 4-6 weeks, which will cover the full lifecyle of common marine parasites. If nothing shows up by then, they should be good to go.
In reality, most problems will show up within 2 weeks or so. Thats how long I qt for.
Honestly, I have so many tanks set up right now that even if I wanted to (which I do) I couldn't set up a 10G for quarrantine. I am hoping to expand my fish room some time soon and get a little (10x) more room. LOL!