The "need" factor is only one aspect. And by that I mean, what do your corals need and what needs to be replaced. The waste export is the 2nd most common reason for water changes, but...think about it. If the cycle is doing its job, your not going to need to export nitrates and nitrites...the cycle will do it. What you will need to export are heavy metals that are deposited into the tank via impure water (top off) and possibly food or additives.
Look at it this way...picture the heavy metals as gravel. Your tank is a 5 gallon bucket. Initially you fill it up..(gravel and water) . Day one...gallon of evap, so a gallon of water/gravel back in. Same on day two. What will eventually happen? The gravel will make the water unihabitable for the fish...right?
As far as what percentage...what is your goal? It should be to keep the tank as stable as possible. This means that the parameters fluctuate as little as possible. Again, that means smaller water changes more often. If you do 5% a week, in the average month, you would have changed 20% of the water! Thats ideal and you didnt have a large impact on the corals and fish!