You see, in general you have 10 bits in serial transmission (1 start bit, 8 bits of data and 1 stop bit). If you have a baud rate error of 10%, the last bit is completely missed. At least 3/4 of it should be in the 10'th bit frame, so up to 2% baud rate error is allowed, but it depends on the serial interface chip you are using on the other side of the line. But 1% will cause no problem.

If you can't do it with timer T1, you can still use timer T2 as a baud rate generator for it has 6 times better resolution than timer T1. So if you have say 3% baud rate error at 9600 baud if using T1, you will have only 0.5% error, if using timer T2.
Using timer T2, I'm running my serial port at 28800 baud & 12MHz crystal. But of course timer T2 is available only on 8052 devices and it's derivates.