Based upon the knowledge that this is common AND I know of no failures of this type, I would say this is not something to be concerned about.I have this same problem, which I noticed while lifting mulch out of my truck's bed. I haven't measured the error yet, but I estimate 1"-1.5". Since I was working with lightweight mulch I wasn't concerned, but it did get me thinking: If I am working with heavy material, will this put undue strain on one half of the loader "system" if the bucket isn't level? (be it the loader frame, cylinder, or whatever)
Countless times I have loaded the bucket to full capacity unevenly. I'm not at all worried about a structural failure, just need to be careful not to tip over sideways.