When accelerating it should downshift once you put a certain ammount of throttle to it. If the throttle increase is very gradual it shouldn't downshift, but once the increase is great enough in a short period of time (varies from model to model, make, year, etc.) it will downshift one, or more, gears depending on the ammount of throttle given.
I would say it's acting fince. If, when shifting, it hits hard into the next gear - jarring you - or doesn't shift at the proper rpms you need to have it serviced/looked at... though it could just be a vacuum line or the linkage, better to question it and have it checked then to have the whole thing crap out on you while going down the road.