If you want to make use of features that look at your bag items, like best in bags, upgrade finder - you’ll want to use the addon to import your character. And you would have to open your bank/void storage at least once with the addon installed. Otherwise, the addon stores your items every time you access those things, so you should be fine.
If you aren’t using those features, it really doesn’t matter.
The other nice thing about the addon is that it remembers the last gear you had equipped for each spec, so you can switch specs on the website and it will know your gear, talents, artifact, etc without you needing to go in-game and activate it, log out, re-import from armory.