No it's not necessary, just faster. It will take at most two days for you to see a tap root and this way you don't put any dud seeds in your hydro setup. Plus you don't have to do anything to the water when germinating this way. Plus you don't use up all that water in your res when you don't need it.

For PH down some people use vinegar or some other form of organic acid but I don't recommend it. It's better to go to the local hydro store and get phosophoric acid. It's not expensive.

Don't worry about your water PPM. It won't cause any problems.