hi
the capacitors are not for filtering, it is used in the charge pump circuit.
that is, to convert the +5V to +12 and -12V
so better use 10uF for performing in the worst cases, but as Steve told maxim ic-max232- may requires only 0.1uF
check the datasheets
i have already made some 10-15 pcb's with
10uF and working well in ISP

anyway best wishes
saji