How many rejected shares one got to be worried about? I'm using NBminer to mine Ethereum on nicehash pool so far I got 78 accepted shares and 9 rejected shares, should I be worried? Phoenix miner latest upgrade lower my hashrate that's why I switch to NBminer, seem more stable, I don't overclocked my gpu and it's temp is below 58°C
What's your network latency? If your value is above 300ms, then 10% of rejected shares is ok.
Overclocking of equipment may also be the reason for rejected shares.
But for a complete test, you need to look at the statistics for a few days.