TO-72 package FET Case pin
The datasheet should tell you definitively tell you whether or not the case is connected to anything on the chip, and if so, where. Sometimes the case is isolated, sometimes the case is connected internally to the chip's ground, etc.

In this case, Note 3,  page 5 of the datasheet from Microsemi says that the case is isolated from the case.

If it's isolated, then connecting to ground is a good bet because leaving it unconnected would be like having a floating ground pour or shield which is not good for noise.

The reason you have difficulty locating other examples is because they tend to be specialized parts contained in these expensive TO packages. Things like exotic optical sensors and amplifiers.