First, my personal opinion on heirlooms. I would make them maybe powerful but not give XP bonuses, and instead for a player, do a stacking 15-20% XP buff for every character you have at max level (that leveled and did not BOOST to max level- and heritage armor shows me the programming is in there to tell the difference).
However, as far as my advice on heirlooms as of 2/7/2019-
1. DO NOT buy weapon upgrades - This is for a few reasons.
• With the other heirloom gear, you will be fine with whatever weapon they give you questing.
• You would have to upgrade the weapon for Legion and BfA content, even though you will get an artifact weapon (waste of gold)
• Mob scaling basically defeats the purpose.
• The weapons give no XP boost at all, so don’t worry about them.
2. DO NOT upgrade the heirloom necklace. It might be obvious, but with the Heart of Azeroth, it would waste your gold to upgrade the neck, it serves no purpose.
- Not sure yet until they go live, but you will have to decide if the Azerite armor is actually better than the heirloom. That is, XP bonus gain v.s. the speed that you can get through quests. If the heirloom are very powerful and compete with the azerite armor, take the looms and get the XP bonus. If having a bunch of azerite armor will make you able to fly through the quests/killing mobs at a rate greater than the XP bonus, then change to azerite gear while leveling and then BEFORE YOU TURN IN A QUEST OR FINISH AN INCURSION OR DUNGEON OR WHATEVER, then put on your heirlooms and finish it up to get the XP bonus at the end.
I think this is the best advice I can currently give as someone with 30 alts. Feel free to add more ideas, and as I said, #3 is in the air right now. Just remember to always switch back to heirloom gear when turning things in/finishing something.