Yes, it's definitely worth powering up the Steem you have invested in, as long as you plan on holding it for longer than 3 months. By powering it up, you can selfvote higher on your blog posts, get higher curation rewards, and get interest on the SP. These might seem small by themselves, but they really add up over time!

So my advice is to always power up as much as possible instead of just holding it as Steem.