At the start of a new season, your casual games will give you a score when you enter the ranked games. But is it better to enter ranked games as soon as allowed, or to delay it by playing more casual games ?
Your casual games won't matter unless you are not Lv10 yet and haven't entered the ranked mode before. Otherwise with the new season you 'll get 1/3 of your mmr from the previous season and therefore get some ranks instantly.
Besides that I suggest just playing ranked from the start, the MMR losses at the early ranks are minimal to say the least, so you are not punished for playing sub-optimal/fun decks or whatever you want to try out basically.