I always assumed Yoda was staying on the sidelines of the Rebellion because of the near extinct nature of the Jedi Knights. The same thing goes for Obi Wan. They were both in hiding waiting for the next coming of the Jedi (Luke) at which time they would allow themselves to be found so they could train him. If they were too easily found before the time they were needed, they could be wiped out by Vader and Palpatine thus assuring the complete eradication of the Jedi. Better to pick your battles than join every one.