Do I need to feed a subpanel with a breaker, or can I use subfeed lugs?
If the wire size from the sub panel is the size that the main breaker in the panel being fed from you don't have to have the breaker but this means if you have a 200 a main breaker in that panel the wire ampacity will need to be that large. Most subs are smaller and it saves $ to install a breaker and feed the sub with smaller wire. Not sure about the comment on not electrocute the line man, is this for a generator or photovoltaic system? I don't see that in the question.

Edit to clarify from comment.
A 125 amp main panel with a 125 amp main braker can direct feed anything larger than 125 A with 2/0 since 2/0 at 75 column is rated for 175a. The run between panels in my area need to be in Ridgid metal conduit because I was replacing the original panel. Some AHJ will limit the distance the panels can be seperated if this is the case. I have done this several times when preparing to replace the original panels (60 &amp; 100 amp to 200) In one case the AHJ did require a 125 amp breaker in the sub, until the service was replaced I thought this was silly because the main breaker in the main panel is still the limiting factor. I did use large enough pipe so when I was ready to convert the service I just had to pull new feeder wire to the meter base so power to the house was only off for a few minutes. Green tag and final prior to meter connection since everything else was already run.