Well that's good to know (that there do in fact exist, in the wild, boards where the internal oscillator isn't good enough for UART at 3.3~5v and room temperature)...
Also, 57600 is a weird baud rate with most Arduino boards - it's treated specially due to some historical oddity that they had to work around; for some reason they turn off UX2 mode for that baud rate specifically. It's probably more sensitive to clock variation at that specific baud rate.
There is no particular concern to running it like that (other than the fact that other individual specimens will require different OSCCAL value, if they need it set at all)