There is as far as I know no strict order of the shields, bit some shields have shorter pins and that can cause short circuit.
Famous in this sense is the old ethernet shield with the big RJ45 connector.
If the XBEE is covered there is a chance the signal is disturbed/weaker.
I think it is OK but a simple test will show it instantly
I have a follow up question.. men you mentioned shorter pins ... the Arduino wireless prototype shield has Long pins ... while the Motor shield (not designed by arduino themselves ) tugs right onto the arduino board ...
is this the case ? will this more likely cause a short circuit ... and why ?