Definitely ask before you PDO bump. I had an FO last month who was consolidating. When we showed at the ramp one evening, we found out he had been PDO bumped off a subsequent flight, without consent, by a pilot on reserve, trying to consolidate. Net result is he's further behind on consolidation, through no fault of his own. I checked with scheduling, and was told there is no requirement for the computer or human scheduler to see a bumpee's consent prior to executing the bump. It is just assumed.

As mentioned by Overnitefr8, I'm all for PDO bumps, just ask. The only time I ever turned one down was a weekend layover at my parent's house.