You should always write a thank you note after a hiring manager takes her precious time to step down from the clouds above to allow you to beg her for a job.
Look, I hate this. Just what are you exactly thanking the manager for? Taking the time to talk with you and consider your application for the job, right? But, what were you really doing? You were taking your time out of your day (and often using vacation time from your current job to do so) to try and solve a problem for the hiring manager.
See, just as much as you need a job, a hiring manager needs someone to fill that job. It's not a situation where the hiring manager gets to feel all warm and fuzzy about being so gracious and kind as to actually hire someone. You won't be hired unless you bring (or rather, you are expected to bring) value to the company.
A job interview should be a two-way street. The hiring manager and the job candidate both have a problem and the relationship should be one of mutual exploration to figure out if you can solve each others' problems. The job interview should be a whole bunch more back and forth and a lot less deference to the hiring manager.
When managers expect thank you notes--or even punish candidates who don't send them--it's an indication that they see themselves as the benevolent one. They aren't.
But it's polite to send a thank you note! Miss Manners probably even said so! It is polite. But the job candidate has used up a ton of her time, often at great sacrifice, to come in to meet with you. Maybe you should be polite and send her a thank you note as well.
Really, I don't care who sends a thank you note to whom. What I care about is flipping the current paradigm on its head. Recruiters and candidates shouldn't be enemies. Hiring managers and candidates should be trying to figure out if the position would be a good fit for all concern. It's a discussion of equals.
When we think of all the things we demand of job candidates, we should realize that they are the ones doing the hiring managers big favors. You need that position filled, and these people are graciously helping you to do so. So, send them a thank you card.