As noted by Cornelius, it should be OK. However, if the goal is to recover the maximum amount of proteins from cells in a small lysis buffer volume, it would help if you recover cells from plate and store cells in a microfuge tube at -80 degree C.
Hey, I go about snap freezing my cells in liquid nitrogen and put them in -80, until I need to lyse them , I also have done that without having to lift the cells from the plate (wash plates 3X with cold PBS first and then freeze plate in liquid nitrogen). Technically freezing them may help with lysis; so once lysed I usually don't btoher with flash freezing and just dump them at -80.
Hey, I am with Cornelius Krasel however if you are looking to evaluate stress protein then I would prefer to harvest the cells first then to store it at -80.
Definitely, there will be loss in protein con. after storing in -80 and also your protein will loss florescent property. Because, freeze-throw method is used as cell lyses.