You only need to declare it once in the start of the program (unless you are changing which uart rprintf are printing to in the program). Nowhere else, when you declare it it stays that way until you turn it off.
If you are just recieving jibberish from your (axon?) microcomputer is probably not connected to the computer properly. I will recommend you to write an easy program with a looping rprintf command and if it prints that, it should also print the other stuff.
Another problem could be that the microcomputer is restarting for some reason, Ive experienced it myself and you'll just recieve jibberish. If you are using axon, what source code are you using?
Ive had some weird problems with the latest source codes released. 1.01 worked well for me, the other ones made my axon reset all the time and print jibberish to the hyperterminal. I would recommend downgrading if you havent tried that.