Importance of Impedance Matching

There's no particular reason to use 50 ohm traces to my opinion. Small traces are rather in the 80 to 90 ohm region, they can be operated impedance matched as well, if necessary. The suggestion to reduce the drive strength respectively using external series resistors are effectively achieving a source series termination, which is a good way for medium speed signals up to 100 MHz ore even more.

Driving an open end 90 ohm transmission line (you won't hardly achieve a higher impedance with your technique) with a 50 ohm source results in a +0.3 reflection factor and a maximal 30% overshoot. That's not a perfect signal quality, but would be sufficient to prevent damage of sensitive devices (e. g. Cyclone III) by overvoltages.