This seems too easy, but...

fBaud Rate = fOsc / N....

So, the oscillator precision needed is directly related to the range of baud rate error you can tolerate.

It is usually +/- 3% (See 8052 Serial Faq #2) for reliable operation. With 8052 you can use Timer 2 for Baud Rate to get a very good selection of N.

Using Timer 1 is when the oscillator frequency needs to be selected carefully. An 11.059 multiple osc is the best.