Don't do any exercise just to do it, or because someone else is doing it. You should be able to justify every aspect of your training plan, or it shouldn't be included.
Understand that many successful lifters do little if any assistance work at all. If you include an assistance lift, you should have a clear understanding of how it is going to help with one or more of the powerlifts, directly (strengthening a weak portion of the lift or strength quality,) or indirectly (building needed muscle mass, improving work capacity so more training can be tolerated). Once you're clear on what you want out of your assistance work this kind of question should take care of itself.
As an example, if you're weak off the floor with your deadlifts but can rack pull a ton, some front squats and russian deadlifts/good mornings might be a useful addition. If your bench is weak off the chest, in addition to more bench volume, some pause benches, overhead work, and maybe some dumbbell work could help.
Just be careful about displacing your work on the powerlifts with assistance. First and foremost you need to practice your skills to get better at them, so don't lose sight of that. Use assistance to assist, not to drive your program.