There is no specific answer - it all depends on the circuit in question and how the component is used. I agree, removing a leg has its risks, so if the component is suspect and cheap, you may as well replace it instead of testing it. Just my opinion, others will chime in.

One of the great things about pinball is that there's usually more than one of each device on the board/ in the game. This makes initial troubleshooting simple. Test the suspect transistor/ diode/ resistor in-circuit, then compare it to it's neighbors. If more than 10% out, replace. Keep in mind that shorted transistors can skew readings on diodes and resistors in the same circuit. Swap out the shorted transistor, then check the rest of the circuit.

Many components absolutely can be tested in-circuit. You generally want to disconnect all the connectors first. Check the pin wiki when in doubt.

The answers you are getting are right on. Sometimes you can, and other times you can’t in place. And when you do test in place, always be aware that your readings can be wonky. Most of the times that will follow logic though. If you are testing a resistor and it reads shorted, either there is another path, or the resistor is shorted. If you are reading nothing across it, or crazy higher than the specs, it’s open. Then just pull it and measure it outside the circuit.

Agree with what has already been said. There are a number of tricks that can
be used to test parts, like transistors and other thing while still in the board
but these will usually only find severe failures like shorted transistors, diodes
or caps.

The more difficult parts to troubleshoot are IC's controlled by the microprocessor.
There is a useful trick for this. There is a mode the 8 bit uP's used in older
SS pins has called 'free run' where it continuously toggles through its entire
address space sequentially. This is perfect for finding address and data lines that
are shorted together or dead for one reason or another. Also finds problems
with address decoders ('LS138's) and buffers. All you need to do to get into
this mode is remove all ROMs. That forces an op code called 'nonop'.
Of course the uP board needs to be getting the correct voltages and the
reset circuit must work.
Steve