I don't claim to be an expert, but i usually visit Greece regularly during the tourist season. My take on this is that while there usually can be (violent) demonstrations in Athens during the winter, these things die out as soon as the spring comes. I have not yet seen any tendency for political unrest during the high tourist season in summer.
Not always. I once arrived at ATH in mid-June only to be confronted with a sudden nationwide passenger ferry strike! (It only lasted a couple of days until the government declared a national emergency, giving it the authority to order the strikers back to work.)
A few years ago I read in a Greek newspaper while I was there that the
kamakia were threatening to go on strike. I
think this was a joke, but I suppose it could have been real ...