I've been through this, too. The OF pitch error is definitely enough to force you to retune your instrument if you want to play along with music. Not a big problem with some wind instruments where you tune all the notes with one change, a big problem with string instruments.
The Rockbox pitch error is so small that you may not have to retune your guitar. If it's noticeable, you can change the pitch in Rockbox on the fly. If you use Rockbox regularly it becomes very easy and intuitive to use, I never use the OF any more. You might try that.