This paper offers a significant evaluation of reader's placement for two wheeled robots to estimate their posture from a lattice of RFID tags. RFID systems where IC tags are installed under/on floors have widely been utilized in recent years as the next positioning infrastructure. The readers' antennas should be properly configured on a robot so that such an environment can give full play to its potential capabilities of positioning the robot. This problem calls for something like guidelines in designing the placement of readers. We constructed a simulation environment using a localization model and evaluated the effects of configurations on positioning accuracy by using detailed computations. Then we obtained the simulation results, which enabled us to identify some useful clues in designing where readers should be placed. We especially focus on a backward sensing problem and discuss how the problem occurs. In addition, a validation experiment using an actual robot verified the validity of the simulation results.