Here is one of the biggest tips I can offer to tutors aiming for 5 star ratings.
The idea is very basic. I don't know of anyone who likes a "Scrooge", a penny-pincher. You are in the service industry, so customer service matters a lot. It's never fun to round down time spent (so you get less pay), but it pays off in the long-run. If you've been with a student an extra few minutes over the scheduled hour, don't try to claim it. It will likely make the students pay more attention to the time than before, in a negative way.
Think of the extra time as a marketing cost; in fact, that is what it is. I have been tutoring full-time for two years now. In that two years, my name has spread very quickly through the surrounding community, both on WyzAnt and off of it. I average around one new tutoring request every day. I no longer worry much about open slots as they often get filled within 24 hours (except college times (i.e. before 3 PM school days), which naturally take longer to fill). I even have a waiting list of people who want to be contacted once I get a cancellation, as well as students who so desire my help that they are willing to meet at obscure times to get an appointment (i.e. midnight and later). Beyond keeping up on the sites and lists I am on, I do no other marketing other than word-of-mouth.
If you want a visual, think of this situation: Say you go to an ice cream place and order a sundae. Three different locations all offer the exact same amount of ice cream for the exact same price. However, the first uses a smaller cup and makes sure it is fully topped off, if not overfilled. The second uses the "correct" size and fills the ice cream to where it "should" be. The third uses a larger cup and doesn't fully fill it. All three places offer the EXACT same deal, but because of how they present it, you have a different reaction. You end up thinking the first is a good value, the second is "normal", and that the third place shorted you.
When you round down the time, in many ways you are even seen as better than the first store: not only are you making sure to "top off" the amount, but you are even doing it with the same "bowl" of one hour.
There is one caution here, and it is the reason I hesitated writing about this. You don't want to advertise the "topping off". It shouldn't be an expectation but rather a bonus. There will be times when you need to keep the session to an hour for whatever reason, and you don't want students thinking that they will always get the "topped off" only to not receive it, as it is human nature to then feel short-changed.
(Note: I use "hour" as that is the most common tutoring length, but it applies to any time amount.)