Unintentional additional percussion provided by my thumb...

I strum with my first two fingers (nails) on the downstroke, and my thumbnail on the upstroke.

Comfy and works fine for me, except when I use anything other than a soprano or banjolele. On those, my thumbnail invariably hits the bottom of the fretboard when I'm on the upstroke, adding a faint, but noticable back-beat.

Is this something I'm doing wrong, or am I just being overly picky (hah!) and should just carry on?

There seem to be many different styles of strumming. The best advice is use what ever method that feels good and produces a good sound. It's all about making some music and having fun!!! I try not to get all hung up in what the "proper" method is.
However, it is also good to try different styles of strumming to produce different sounds!

Hi Chap,
I have the same issue with my thumb nail on upstrokes. What I do is slow down and not allow my thumb to hit the fret board unless that is what I want. I don't have it down 100% ...yet. Adapt it and use it too. Percussive sounds are cool if done when you want them.
DAP