Grounds on an Evaluation Board
In general, yes, it's probably safe to do so in most cases. Here are the exceptions that I can think of:


High-current systems. Even if all of the grounds on the board are connected via a ground plane or some other conductor, there is still some resistance between any two points. Typically it is in the milli- or micro-Ohms and thus negligible. However, given enough current flowing between two points, an non-negligible voltage drop can develop, according to Ohm's Law.
Absence of a ground plane. Most circuits are designed with a ground plane. If the circuit lacks a ground plane and is a rats nest of wires instead, it probably isn't safe to assume that all ground points are electrically equivalent.
High-frequency signals. The ground return path of high-frequency signals doesn't follow the same behavior as low-frequency and DC currents. As such, the measured voltages from two different ground reference locations on the same ground plane may give very different values.
Isolated grounds. Certain applications may call for multiple grounds that are electrically isolated. A typical example of this is a transformer that plugs in the wall outlet. The ground reference in the device it powers will not be the same as the building's ground.