I certainly wouldn't recommend that. There are two factors to consider:
1. the voltage you are sensing is produce relative to ground at the sensor and measured relative to ground at the MCU end. If the ground at the sensor already has a voltage on it from elsewhere, or if it is floating, the potential difference between the MCU input and MCU ground could stop it working or give intermittent results.
2. For the same reasons, a difference in ground voltage at each end could produce a voltage at the MCU sufficient to permanently damage it.
Always make sure the grounds are connected directly together and with an impedance as low as possible to minimize ground voltage differences.
Brian.