Is it rude to request a read receipt when emailing the professor?
Unless you have reason to suspect that emails are not being delivered successfully, email receipts are pretty much useless. Here's why:


Not all email clients support read receipts - Mail on OS X doesn't, for one prominent example. If some of the previous emails you sent requested a receipt, and it wasn't returned, obviously either the prof's email client doesn't support it or he didn't bother clicking that button.
Unless the receipt is returned, you can never be sure if the email was read or not.
Unless the email is replied to immediately - which is unlikely if the reply will take some effort - it can easily be forgotten about. Receiving an email receipt is no guarantee that you'll get a reply.


If you have no response to an email after a reasonable period of time ("reasonable" can vary between 3 days and a month or more, depending on what is required), just send a quick reminder email.