Great question! It took me a while to work this out too 
As a general rule of thumb, I give five stars.
If there is something worth mentioning, I include it in the written review in a friendly and factual way, perhaps with some context.
For example, keeping a home spotless in the Pacific, without windows and with pets and all kinds of insects coming inside, can be challenging. I would mention those circumstances, but I would not deduct stars for cleanliness. If there were piles of poop in the house, however, I would lower the rating and describe the situation factually.
If the house was not particularly clean but the hosts had left a huge welcome basket, I might deduct no stars or perhaps one, while still mentioning the cleanliness in a kind and factual way. Some sitters would not mind starting a sit like that, while others certainly would.
Most importantly, if you are considering giving fewer than five stars, check with the community on this forum first. It can help you put your experience into perspective.
In general, pet sitters want to be treated with mutual respect, and TrustedHousesitters should be a win win arrangement. Some hosts apparently treat pet sitters as staff, and that is not acceptable.
Something else we do is mirror the host’s language. We repeat their requests in our own words and often use the same words and expressions they use. We also clearly confirm that we understand what they are asking and that we will follow their instructions. Speaking the same language in this way makes communication easier and helps the host feel heard and understood. It is something you rarely see mentioned anywhere, but it can make a real difference to building trust and having a successful conversation and sit.
First time sitters and hosts can sometimes be challenging, particularly when they have no previous experience with reciprocal arrangements such as house swapping. A first timer may even withdraw at the last moment. Fortunately, I have never experienced this myself, but I have friends to whom it has happened several times.
It is also very important to keep hosts updated with photos and videos of their pets, whether they respond or not. It may not be an official obligation, and some pet sitters apparently find it difficult, but we always do it, even when the host has not specifically asked. We also make it clear that they do not need to reply. We simply enjoy sharing the lovely moments we have with their pets.
We also always have a video call with a potential host. It gives us a better sense of who they are and allows us to discuss any additional requests that we might not feel comfortable accepting or that could create a risk for us.
For example, if a property is for sale, a host might ask us to prepare the house or grounds for viewings. I used to work as an estate agent, so I know how important the presentation of a property can be. If I do not know the host, I would probably decline that responsibility. Imagine that the selling agent later says the house looked untidy because one small piece of paper had been overlooked. If the host then believes this affected the sale, it could have serious consequences for our review.
If I already know the host or have sat for them before, I have a better understanding of how strict or flexible they are and how they respond to ordinary human mistakes, such as overlooking one small item in an otherwise spotless house or accidentally breaking a glass. In that situation, I might be more willing to accept the request.
That is another important reason for having a video call. Some additional requests are perfectly reasonable and easy to accept, while others carry responsibilities and risks that make it wiser to say no. This is part of my personal assessment and something that is rarely discussed or written down.