Hitching is never entirely safe, and we don’t recommend it. Travellers who hitch should understand that they are taking a small but potentially serious risk. People who do choose to hitch will be safer if they travel in pairs; they should let someone know where they are planning to go. In particular, it is unwise for women to hitch alone; women are better off hitching with a male companion.
Some parts of Greece are much better for hitching than others. Getting out of major cities tends to be hard work and Athens is notoriously difficult. Hitching is much easier in remote areas and on islands with poor public transport. On country roads it is not unknown for someone to stop and ask if you want a lift, even if you haven't stuck a thumb out.