Everyone is different.
I've been taking Niacin under prescription for a few months now (500mg extended release). I haven't noticed an increase in tinnitus associated with it. I've had spikes before and after I started taking it, but overall my tinnitus is the same as it was before I started.
There was a time when I was taking Niacin before bed to help me sleep. It was a 250 mg time-released tablet. Then one day, out of nowhere, I had a huge spike. I stopped taking it after that.
Here's my take: I tried Ginkgo Biloba in the very beginning of my odyssey, which was supposed to help tinnitus because it improved circulation. Well, it made my tinnitus worse.
I believe Niacin also improves circulation, which is responsible for then "Niacin flush" phenomenon.
So it would seem that improvement in circulation can actually increase tinnitus volume.
But there's one caveat that I keep in mind. I've read that sometimes things that exacerbate tinnitus in the beginning, can eventually be the thing that will improve it in the long run. I recently started with my that Ginkgo Biloba again, and have not noticed any spikes (I'm only taking one capsule however instead of four I was taking last year).
It calmed the next day. Actually it calmed way down my baseline the next day. I think Niacin helped? I might take it again but maybe microdosing it won't make me spike.