Why don't I need to consider trace width for a high speed oscillator crystals?
A 100MHz oscillation has a wavelength of 3 metres and, a general rule of thumb is that you don't need to consider track length impedance matching if the trace length is less that one-tenth of the wavelength.

So, if you are using a 100MHz oscillator and it is sited at least 0.3 metres from the chip it feeds, then you ought to consider doing something; either shortening the track or impedance matching.

At 100MHz the output will probably be more sinusoidal than square so there shouldn't be a problem AND if you try to consider the sq wave harmonics of a much lower frequency oscillator you'll still conclude that things only start to become a problem when you have ridiculously long tracks and frequencies above 100MHz (half theory half rule of thumb).