I tend to launch in and it becomes apparent at some point which market the piece is leaning towards. Then I feel extra confident, because I know where I'm heading. I don't usually decide in advance that I'm going to write a story for a particular publication unless there's a competition with a strict brief about style and content. For example, Writers' Forum magazine have run competitions to write for Woman's Weekly and People's Friend, for example. These are fun to enter for a change.
It adds variety to have rules occasionally and it's good discipline. Then you can enjoy the freedom to write however you like afterwards.