That question has been asked several times on here and the consensus is that it is frowned upon. FAs do not allow saving of seats, so if someone decided to sit there, there is nothing he could do and the FA will not back him up. Plus if they catch him trying to save seats they may say something to him anyway.
This. The last flight I was on the FA's made an announcment this is open seating and people can seat anywhere another person was not. That people needed to remove bags from seats so people could sit it was a full flight and things like that delay boarding.