I'd probably wait a few hours before testing the water after a water change. In my 100 gal. tank, I vacuum all the gravel on the bottom, but then I have a lot of filtration on the tank and it's got 2 oscars and a common pleco in it, so I guess the answer to your question is if you have a lot of filtration, vacuum it all if you want to. If you do that, and your water gets cloudy after doing maintenance, then you probably need to bump up the filtration, or vacuum less of the gravel each time. A good rule of thumb is to vacuum no more than half of the tank so your bacteria isn't too disturbed, but like in my case, my fish are huge waste producers, and I have extra filtration on the tank, so it's not an issue.