1-100 go straight dungeons nothing else (if you can get insta que via heals/tank)
If dps 1-100 dungeon as much as possible and questing in between. As long as your full heirloom dungeon will be the way to go.

Its to 100

As in you can grant levels up to 100? Good to know. Thanks!

buy legion get a free lvl 100 boost

If I RAF and they buy legion can they grant 50 levels off of their level 100 boosted toon?

Level Granting

For every two levels your recruit gains while the accounts are linked, they can grant one level to one of your characters. Your friend must log in to the character with grantable levels, right-click on your character's portrait, and select Grant A Level. The Grant A Level menu item will only appear when:

The characters are in a party on the same realm and faction.
The characters are within 100 yards of each other.
The character granting the level is higher level than the character receiving the level.
The character receiving the level is level 89 or lower.
The character receiving the level has not disabled experience gain.
Only levels earned through normal gameplay count for grantable levels. Levels earned through Character Boost do not apply.

taken from battle.net