The quick answer to this is no. You do not need a scale. It is very easy to learn to eyeball the starter when using it in a recipe and eyeball the flour at feeding time. If you are uncomfortable with eyeballing it, then a scale can come in handy when measuring out by grams during feeding time and bread-baking time.
An example of eyeballing for feeding the starter would be to pour in as much flour as there is starter, and it is okay to add extra just in case. Then you would pour water in until you reached a muffin-like texture. However, the San Francisco starter may be more like a thick pancake batter.
Example of using a scale would be: 50 grams of starter: 50 grams of flour: 50 grams of water.
