>so that the "back" button is at the top left corner of the screen

"Back" is used so frequently that I consider any mouse lacking a side button for "back" to be incomplete. You don't need to put a high value on your time for upgrading your mouse to pay for itself within its service life.

On the other end of that spectrum I cannot _stand_ back button on the mouse and always disable the feature immediately because it is often too easy for me to click those fiddly side buttons without meaning to (either while grabbing or while re-adjusting my hand position on the mouse)

So I usually only map middle click to such a button, as that button is at least 90% of the time relatively safe to accidentally trigger.