I leave the ferrule and nut on the column when I remove a column from the GC and replace ferrule, nut, and all when reinstalling a column.
Because ferrules deform when used, I always discard a ferrule after removing it from a column. You need to re-open the hole in the ferrule to insert a column and then compress the ferrule back around the column to obtain a seal. And, depending on the hardware being used, tightening the ferrule may extrude bits of the ferrule into an inlet or transfer line, leaving less material to compress around the column. Keep in mind that after long use a ferrule may become sufficiently compressed that it needs to be removed from a column and discarded - even without ever removing the column from the oven. I'd rather toss out a few extra ferrules than fight air leaks in a mass spectrometer or lose runs. (Leakage may be more obvious with a GC/MS system - as you look for the very low levels of air that can come into the system - and I do primarily GC/MS.)
On reuse of nuts - examine the nut before installing it in the GC. IF you can see that threads or flat surfaces for the wrench are becoming worn - discard the nut. This is a more common problem with a transfer line nut for MS than for other nuts. If a nut becomes jammed in place, the removal process can be difficult, time-consuming, and expensive (in your time or in the cost of repairs to other parts damaged in the process of removing the nut).
And after intalling a column, check for leaks. Use a leak sniffer of some kind for connections around inlets and detectors that run at or above atmospheric pressure. Be sure that mass spec systems are leak free after pumping down. Do not assume that just because you can see pretty big peaks, the instrument is leak free. Carrier gas leaking through a loose ferrule may not bleed all of your sample away - but it does introduce an uncontrolled split - which will become worse over time.
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