Will I keep my league placement if I don't play any more than the five placement games?

I just played five really good matches and got placed in a high league. If I don't play any more (to risk losing more games and dropping) will I stay where I am or drop down in the rankings? I'm aiming for the 'Finished a season in a 1v1 Master Division' achievement.

Solution 1:

If you only play the 5 placement matches, you'll stay in the league you were initially placed in. But your position will change based on points in the division of said league, the players that accumulate more points than you will overtake you in the division ladder.

There are really no negative effects associated with going inactive. You don't get demoted from leagues due to inactivity. Your MMR does not decrease. Your points do not decrease.

From the Comprehensive SC2 League and Ladder Guide on Team Liquid.

So, you cannot drop down a league for inactivity, exception made for the Grand Master league, which will periodically remove inactive players.