Updated: June 11, 2018
It's important to keep an eye on the repin rate for your group boards. When you've just gotten into a new board, give it about a month to see how your pins do. Generally, you can either leave a board or remove any board that is less than .1 repin rate from your campaigns. You can find the repin rate for any of your boards in the "Board Performance" report on Boardbooster.
This decision is really based on the niche and the group board's performance. Some niches/topics perform really well so you can have a repin rate (according to BoardBooster) of over 1 while others just can't seem to get above 0.4.
If you feel the pins in the board are relevant then don't worry so much about repin rate (unless it's 0) and just make sure the pins surrounding yours are relevant.