No, imagine a game like this.
You finish one Region with your full leveled team ( after elite 4 around 60ish)
Then some post game in this Region, now the postgame story drives you into the next Region, fighting agaisnt those postgame enemies you will used your "champion team". but a Professor will tell you in these Region are even more Gyms and Pokemon, you get a second roster of team, the rookie team.
You will fight through the Gym's with your rookie team, after you obtained the 8th badge you can mix teams, throughout the new Region there are several Trainers in the wild who will differ which team they will use. Rinse and repeat for new Regions.
Edit: Also everybody who said Sinnoh was a good region is wrong.