I found it more productive to have a second job on days off then to pick up open time, especially on first year pay. Not only do I make more money than most open time trips, at the end of the day I am at home and actually sleep in my bed at the end of the night. Its hard being gone for a 4 day trip then pick up another 2, 3, or 4 day trip only to be right back to work for your next regularly assigned 4 day trip. All just for a few hundred bucks.
On top of that, you could learn a new skill. I always remember my medical is a heartbeat away from being yanked. That keeps me motivated to pursue non aviation sources of income.
Of course this debate could change the second I become captain and we get a good contract with any kind of soft pay.