1 GHz has a wave length of 30cm, which is much longer than the trace from the transformer output to the chip's clock input.
Actually, the wavelength depends on the medium. If you are using a
microstrip, stripline, CPS, CPW,etc. then the wavelength is NOT 30 cm.
Roughly it is about 50 to 60% of the wavelenght on the air. So, I would
say the wavelength is about 18cm. A 1/10 of that would be 1.8 cm.
Now we can say that anything greater than 1.8 cm should be consider
a transmission line and needs matching.
For other side, for a 1 GHz clock the cycle is 1 ns. Assuming that 5% is
for rise time or 50 ps, the bandwidth to consider would be about 7GHz
(BW= 0.35 /(Tr)). In others words, the wavelength to consider would be
18cm/7= 2.57 cm and 1/10 of it would be 2.57 mm. So, again anything
longer than 2.57 mm is a transmission line.
Other consideration, for BALUN (or transformer) made inductively the
bandwidth is roughly 750 MHz at the best. So, I doubt that the main problem
you will face is the matching but the transformer itself.