In Oregon, one can only file for divorce in a county where one party or the other lives.
Although the rules which govern divorces are the same throughout the state, sometimes there can be procedural differences from county to county. These can include the ease with which in person hearings can be had on initial matters, prior to the main trial (which may be months and months away). These can also include procedures and options relating to court sponsored mediation, court scheduling issues, and the like.
In addition, of course, if your lawyer is more familiar with the court system in one county, or is able to get to court in that county without a long drive, it can be important to be the first person to file in a divorce, so that your case will be heard in that county instead of some other county where your lawyer is less familiar with the procedures, or where your lawyer would have to drive a long way for any hearings or for the trial itself.