The baud rate generator will be accurate. If the formula says you will get 115KBaud with a 20Mhz oscillator, you will.
If you don't get 115K baud, then you aren't really running at 20Mhz, or the receiving device has an issue.

I mentioned in another post how to measure just how fast your chip is running - without an oscilloscope.

(Turn LED ON)
For X = 1 to 60
Pause 60000
Next X
(Turn LED OFF)

Wait one hour. If your LED goes off in exactly 60 minutes, your chip is running the speed you think it is. Using a
calculator, you can measure the actual frequency to about 0.1%

A useful tool is BRAY's TERMINAL program (Google it). Free, and it lets you set odd baud rates, like 113Kbaud.
or 120Kbaud.