If you achieve a decent Hashrate, not really necessary. The pool does a pretty good job at that.
You may can squeeze out a few GH/s by setting it, but there is some testing needed, as each miner has it's own chracteristic, also the speed and reliability of your network connection has an influence . E.g. I have set SP20 to 1792 and S5 to 1280 at the moment. But these number may vary widely in your setup.
Not much of a difference if I let it to auto, main advantage that you are up to speed faster e.g. after a pool restart.