EXCEPTION: If you have Mining and only use Jewelcrafting to craft stone statues (for additional in-combat healing), then this may be a useful tradeskill. However, like Engineering above, it should not be a leveling focus and should only be used to assist with power leveling. Doing any more than this will become costly and should not be undertaken during the power-leveling process.
As late as possible. Early on, you’ll want to follow the missions in Battle for Azeroth right up until you get the option to send a follower on a two-hour long quest. As soon as you get there, stop doing the War Campaign missions. You can come back to them once you have reached something like 119 and a half and finish leveling with those if you want. If you don’t use these missions to cap off your leveling journey, then they’re the first thing you’ll want to do once you reach max level.