Thats tottaly up to how you want your game to be.You can even have both,just set the random encounters on the map,and make events with special monsters that appear on the field screen!
For example,have random encounters with regular creatures,like bats,and bees and what not,and have a goblin walkin around the map,and when you talk to him he'll say like "i'm gonna have you for breakfast!",and start a battle...