I started off my unRAID server with High Watermark and all disks included in all shares. It's a good setting for a new user to start with, and remains a good setting for users who just like to use their server rather than tinker with it.
That said, I came to three conclusions. One, I actively manage my server - disks aren't going to fill up without my knowing. Two, I didn't like having files in a share scattered unnecessarily across multiple disks. Three, while having equal amounts of data smoothly spread across all disks seems appealing, it's actually a pain to manage.
So, at the recommendation of some folks here like @SSD I switched over to having shares "include" only the disks I want the data on, and all shares are set to "Fill" each disk (with a nice big minimum free space). I also use the diskmv/consld8 utilities to move data from one set of disks to another when infrequently needed. Warning, this is a more advanced approach that requires you to actively manage your server - one disk could fill up while you have space available on others. But I prefer it so far.