Hey there,
im coming to the end of my fourth week of growth on my hydro set-up with a flood n drain. I don't think you can go wrong if you use rockwool as your medium...it makes life a hell of a lot easier when transplanting seedlings to the main grow room etc...so much less stress on the young plant. Just start your seeds off in dampened prepared rockwool, then when they're ready to go into the main grow setup, just prepare bigger rockwool cubes with a hole in so that the smaller ones can fit straight in to them....fingers crossed, a very simple and beginner friendly procedure!!!
Im a first timer that didnt really have much of a clue, and look how mine have turned out so far!!!
As for your PH question, Id say its best to keep it clean and just check it as often as you can...one thing you cannot afford is to ph levels wrong or the plants can suffer. If you leave it in all the time, it may become clogged up or something, id personally prefer not to take the risk and clean it after each use.
Hope it goes well for ya
MNSN