I'm not familiar with Richman's book. Does he say anything about this?
So far as I'm aware, fingering isn't considered to be important at all when you're doing sightreading practice. The whole point is to build and reinforce the process of seeing notes on the score and playing them in an automatic, reflexive manner. See it, play it. In the eyes, out the fingers. Don't pause to identify notes, and press onward without correcting yourself when you make mistakes. Don't think about it.
Sightreading exercises are intended for this very specific purpose—developing an unconscious and instinctive connection between the printed notes and the keyboard—and the material you use should be played through only once. In the event you inadvertently employ "bad" fingering, it won't become ingrained because you're not doing the kind of repetitive practice necessary for learning a piece of music.
Over time, you will become more adept at "looking ahead" and anticipating what comes next. As you become a more skilled pianist, the sound fingering principles that you absorb from the repertoire you learn will tend more and more to prevail naturally when you sightread. My advice is don't worry about it. In fact, don't even think about it!
[Edit: csharp minor posted at the same time I did. Of course, you should look over what you're going to sightread beforehand and think about what it entails. When I say "don't think," I mean don't stop to think once you've commenced; just press onward.]
Topic: Learning to sight read - fingering