Strictly personal preference. You can get a Molle pouch for anything, so it's up to you to decide what you wish to carry and where.
If you need a Molle cell phone case, and you want to be able to draw your phone in your off hand while pulling your pistol from your cool Molle holster, you have but to adjust them as such.
That's the beauty of this system.
Likewise, you aren't obligated to use your Molle canteen carrier if you don't carry a canteen.