There are tables in the PIC datasheets which show calculated error values for different baud rates. I would check them to see what values you are likely to get.
I have used the internal oscillator with software and hardware UART and never had a problem. If you are writing a serious app then add some CRC or error correction data into your protocol and you will be fine.
As a general note about the internal oscillator, if you change speeds (including the first time you set the speed in your program) don't forget to wait for the oscillator to be stable. You can do this by checking the IOFS bit in the OSCCON register.