No need to treat them with care, they are sitting in oil so don't seize. However i would use care when tightening...BLB exclusivley metal plugs according to this which I think is audi workshop manual.
Probably still need to be treated with care mind you...
The glow plugs are to aid starting that's all,if the end drops off one or they become loose would be the only way they can effect running.Just Ben looking at different plugs listed for blb, and there is a big range of differences and it looks a minefield. Some list ceramic but as sdha points out this seems to be wrong,some list 4.4 volt,7 volt and 12volt so I would say its easy to get it wrong and blow a full set.