When you’re the one attacking, then it doesn’t really matter - apart from making sure that your minion summoners / healers that only impact the adjacent allies… actually have allies on either side of them.
In defence though, it is more important to be mindful of who you’re placing where (see image), for a multitude of reasons. as you mentioned, you might want to have a healer as your tank, or you might want to keep some of your other heroes back. You can also use a custom formation (once unlocked) to alter who your tanks are.
The other key reason for being aware of placement, is because the AI will automatically cast specials from left to right, so you want to make sure you have good synergy there too!
Here’s some other useful guides that might help too!
Here’s some other topics that might help, and if in doubt, check the other linked posts beneath them. It might take a bit of searching to find the answers you need, but a little research goes a long way! I also believe that @OldCynic has a really useful blog post about jargon busting and defence positioning 
Good luck, and keep the fight up! 