Ya mainly 2-3%, any firmware that asks for more should be avoided, and I don't think it "periodically changes", at least not for Vnish, they claim "Parallel mining"
I used 'periodically changes' as a simple way to describe it. They use a stratum
security hole 'feature' called #xnonce to seamlessly redirect where some shares are received from/sent to (DevPool) without actually resetting/restarting a miner like changing a pool would do.