The question is not "do you use interrupts for your software UART" (you should) but "do you use interrupts". If you run the software UART with a timing loop and an interrupt comes in - say from the hard UART - your time will be off by the time it takes to process that interrupt. If you run the software UART with interrupts and you have not set the software UART interrupts as the only ones having high priority, you timing will be off sometimes. These offsets will be absolute and thus as a percent of error be less at 2400 than at 9600.

Have fun,

Erik