Well that is one way to do things. Most "production" tanks, if you take the rims off, have the sides on top of the bottom as well.
What makes strength for the seams, is the width of the seam. Either way works. However, you can get a wider seam with sides around the bottom than you can with sides on top of the bottom, because you should be using a thicker bottom panel. Either way though, you should have an internal bottom euro brace if going rimless. It is a fine technical point.