If I'm going to use Burton water salts in my brew, should I add the package of it (made for 5 gallons) to my wort before boil, or do I need to add it to the total volume of water before mashing?
The salts are used during mashing. Use them in the total water volume before mashing.
I would say to NOT use them unless you know your water profile and know what it will do to it.
I do know my water profile, very soft, so they work.