I agree with the above, to the extent that one shouldn't assume that a bb cup is sufficiently tight even if it seems tight.
But an alloy cup should always be pulled and have threads greased when bottom bracket service is done.


In other words, leaving a steel cup in place is ok as long as some real force is used in trying to remove it (and failing), or if some serious tightening torque is applied just in case the cup isn't really in there as tight as it should be.


Factory-installed French and Italian fixed cups usually were tightened very well at the factory, quite unlike those on many Japanese-built bikes. I often leave French bb cups in place, as is, when doing a bb rebuild, but I still test the cup for looseness using some real torque while the crankarms are off.
There are wrenches that can secure the fixed cup with a road-double crankset still in place, and I have had to tighten a loose fixed cup in this manner a few times, but the bearing adjustment at the adjustable cup side definitely needs to be revisited after the fixed cup is re-tightened in these cases. Some adjustable cups and lockrings can be re-adjusted and/or secured with the crankarms in place, using appropriate tools for the particular bb configuration.