When I first got my 30D I found that frequently my photos had a bit of a lean to them. It got to the point where I though the camera had a misalignment somewhere so I fired off a very simple test shot - just aiming at my TV, which has nice, straight, squared off edges. When performing such a test you need to be centred on the TV, both horizontally and vertically, otherwise you will get a skewed effect of converging lines if any corner/side of the TV is further away from the camera than the other. When I paid attention to what I was doing the image produced by the camera came out just fine. So my problems were 100% user error and 0% equipment.
The thing is, when shooting an SLR there is so much to think about - so many settings and options, composition, lighting, flash, exposure, focusing, depth of field and it is easy to let something slip. That slight tilt is a sure sign of dropping the ball on one of the items on the mental checklist.
Another thing to note - if you are shooting something with a lot of vertical reference points, such as indoors with walls, door frames and window frame, if you tilt the camera even a tiny bit up or down you will get converging verticals. Tilt down and the verticals will lean out at the top. Tilt up, such as when photographing the outside of a building, and your verticals will lean in at the top. If you want to keep your verticals vertical you must keep the camer/lens pointing level.
With that in mind, if you are shooting and the camera has a tilt up or down then be very careful how you line up your imaginary horizontal line. Given that verticals on the left or right will be leaning either in or out, you should not use verticals at the edge of the frame to judge your alignment. You need to find a vertical near the centre of the image, which will lean neither in nor out, and/or watch the verticals at both sides and make sure that their lean is equally balanced. I did have a habit of straightening my right hand vertical, when shooting indoors, and discovered only later that my left hand side had a massive lean to it.
Here's an example. The wonkiness is because I had the camera at my eye level but, as you can see, the centre of the frame is at the bride's waist level, which is where the camera was pointed, i.e. downwards. See how the right wall edge is pretty much vertical but the left is looking pretty skewed.