I agree with Wirless, most pilots use the flight instructor position to build hours and experience to then be put in the position to apply for right hand seat jobs. This is not always the case however, a lot of pilots just instruct and find great fulfilment in that..
Unless you are one of the very lucky few and get on a full sposorship programme, you will more than likely become a flight instructer anyway..
My advice, dont worry about it too much, just get the ATPL's done, get some air time under your belt and go from there.
Rotormad