LVPECL termination impedance
As long as the receiver termination is good, the reflected signal will be very small, and therefore there won't be much reverse travelling signal to reflect off the source. So often the source is left unmatched in PECL sustems.

It's also possible to terminate at the source but not at the load. There are application circuits for this in, for example, On Semi App Note 8020. In this case you send a 1/2-amplitude signal down the line, and the near-100% reflection at the load causes the load to see the full 800-1000 mV amplitude expected in PECL.

If you do choose to terminate the source you should use about 45 ohms, not 50, because the PECL output has about 5 ohms equivalent impedance.  

If you terminate both ends, the amplitude of the signal reaching the load will  be cut in half. This may be acceptable if the load is something other than another PECL gate, and you want to absolutely minimize reflected signals on the line for some reason.