Basically, it all comes down to common sense and common courtesy. The problem is that 'common' isn't so common anymore.
Sure you can recline your seat. But please, check behind you first. If you find that the tray is being used or the person is bending down to get something, or is very tall..recline slowly. Do not just slam backwards. People do that to my dh all the time...and at 6'4'', he has very long legs. He is always being slammed into...the person reclining in front of him can't figure out why the seat is being hindered. Oh, it's just dh's knees.
If you are courteous, then you will be met with courteous responses.