Flat is perfectly fine when you're not repeating sheets, and the interconnection is more general than tree-like.
In a flat design, you can use Port symbols, or use Off-Sheet Connectors. Wires named with the same Net Label will also be connected, but you should try to avoid that because it leads down the slippery slope of graphical netlisting*.
*I don't know if anyone calls it this, but consider: a netlist is a plain-text representation, naming every pin of every component, and which nets those pins go to. Utterly useless for seeing how a design is connected up. Now consider a schematic design, where components are placed wherever, and each pin has a net name on it, no wires needed...