"Thank you" emails for job interviews?

I had a job interview this morning where 4 different people interviewed me in a row. It looks like I will probably have another interview, probably next week, with 2 other people. Is it appropriate for me to send "thank you" emails to the people who interviewed me today? Should I do that now (like today or tomorrow), or should I wait until all of the interviews are done next week?

Thanks for your help!