are your loads including unevenly heavy items? it's not just a matter of levelness but a matter of the same pressure on all four feet. Be sure that no foot has a noticeably lighter load on it. To check, pull up on each corner and see if one corner rocks a bit. If the tile is uneven, you could glue down one or two (scwed together) sheets of 1-1/4" Sturdifloor and carefully level the washer on top of that.

Failing all of that, make a padded holder bar that goes at the back top of the washer to reduce it's tendency to rock. that's what I did and it helps a LOT to reduce out of balance issues as the washer starts to spin. once mine gets up to a pretty good speed, it's always smooth no matter how out of balance it may have been at the start of the spin.

Dave - please explain the padded holder bar

construct a wooden bar the appropriate length for the washer, and attach roughly 6-10" long ends to "capture" the top of the washer. Glue & screw the end pieces to the top part. Make sure it is padded (indoor/outdoor carpeting--the thin stuff) so as not to scuff the washer. place it at the rear so as not to interfere with your use of the rest of the top of the washer. I covered mine in the same gray color material as the gray of our floor. Looks great, matches the gray washer and keeps it from starting to rock a lot on out of balance loads. If you aren't all handy, forget it. :)