Importance of Impedance Matching

For clocked signals, I would just set the MAXII outputs to slow slew rate so they don't ring too much. At 20MHz, as long as your traces are not ridiculously long, they will settle before the next clock edge.

For asynchronous or clock signals, you should make sure the traces are properly terminated. For point to point signals, just put a series resistor at the driver such that the sum of the driver impedance and the resistor equals the trace impedance. For multidrop signals, run a single trace to all destinations. Parallel terminate the trace at or near the final destination.

Note that the above are general guidelines which should be followed regardless of trace impedance. There is nothing magic about 50 Ohm traces.