A flat table is a big factor in getting accurate cuts when routing. You can easily check the flatness of the top with a straightedge. You’ll want to check
the center of the table, from side to side and front to back.
The problem with even a slight dip is that workpieces can “bridge” the hollow spot without contacting the table. That means a shallower cut in the middle of a long workpiece, which can result in joinery that doesn’t fit quite right. If you find that your table isn't flat, a simple solution is to attach bracing under the table to keep it flat and rigid.