Usually you apply knowing that you could be posted to any base. So you really have to be prepared for that. If the airline in question has more than one base, it may be possible to have a first choice but it pretty much means nothing during your early stages.
My first base was 5800NM away in Manila. Obviously I knew that and prepared myself for it-including the $1800 pcm salary I had to accept. I grabbed the opportunity with both hands, especially as I knew that my cost of living in Manila would be the equivalent of me earning £100K a year living in London!
I agree once you get that first job, you must never let go of it, especially during the early stages. Within 18 months, I had the possibility of going to Continental Micronesia based in Guam.Luckily I was in a position not to take it.