I dont belive that lobsters need QT infact I am sure that most medicines would probably kill them especially copper.
I could be wrong but I am pretty sure...
Personally I think it is unlikely but not impossible. Even though they may not be infected with a disease they could still carry parasites that could harm other things in your tank. Doesn't hurt to isolate them in a QT for a few weeks to make sure they are eating and hopefully any nasties to die off.