Quote Originally Posted by nitewalk View Post
Your world really a bit slanted. I think the tripod will help even if you are using a faster shutter speed during daytime, so you can slowly align properly. Otherwise, you could get a hotshoe bubble level and learn to align the bubble in the middle, that could help. Some cameras have in camera level, not too sure if 6D has it. Those, while not totally accurate, can help to reduce the tilt.
Alternatively, shoot wider a bit, then correct the tilt in post. You should always try to make sure it's as straight as possible while shooting, but don't waste too much time doing that. The more time you spend trying to make your camera perfectly level, the less time you have to think about other elements of the shot (composition, lighting etc)

Also when using a tripod, get your composition right before you set it up. Otherwise it's very easy to get distracted and leave the camera in the same spot for a long time and get only one composition. I did this many times before and only realised my mistake when I got home.