I live in a 150+ year old house. The floors actually bounce. As long as you dont have "serious sloshing" goin on I don't think you have anything to worry about.
I also live in an old house, but the wiggle really scared me. My dad anchored my stands to the wall, making sure to use a stud finder. The next week we had a small earthquake, and I was really grateful he did that for me.
This is pretty common, the flor flexes a bit and you get water movement in the tank. One way to reduce this is to widen the footprint of the tank stand. Take a piece of 5/8" plywood and cut it as long as the stand but 2' wide (for a 55G) cover the plywood with decorative tiles or linoleum. then next time you do a major water change get a couple of friends to help you slip the plywood under the stand. Doing so will reduce the swaying considerably.