It really depends on the situation for me.
Ill switch back and forth in different scenarios but primarily I'll keep it locked.
Quite a few occasions when action starts going on suddenly and i leave it unlocked, ill twitch mouse too much to the sides of the screen and end up looking somewhere off map in the middle of a fight.
Red Side in lane, unlocked occasionally to get more vision of the lane. Global Skillshots unlocked. Viewing other lanes for more than a split second, unlocked, otherwise click on map for temporary view. Anticipating enemy movements unlocked. A lot of the mapawareness you need can be gathered from right there, on the MAP itself. The only things you cant gather from looking at the minimap, is where global enemy ults are coming from, and certain Fog Visible things, such as someone trinketing a bush nearby.