Not sure if this is new or if I've just missed it before, I did make a request quite a while back....

Sales person can be made "Mandatory", though it's not in the place I'd expect it - in General preferences, next to "Add field for Sales person" !

It can be found in Preferences | Sales orders | Field Customization | Hover over the "Sales person" field & a red dropDown will appear where you will find this option !