This Sunday I plan on bringing about 4 pre-made armies of 500pts (or close to) each, already sorted into their own containers. Is this the preferred method?

There isn't a preferred method that I am aware of. Our first couple of meetings, I grabbed some figs based on what the other guys were playing. The last time, I played one pre-planned army, for the sole purpose of killing Bill's vamps. It worked well against other figs though, too.