If the amount at stake is not large, think carefully after your emotions have calmed down.
Will they Sue me Back?
If the other side has claims against you, they may cross complain or counter sue you. This may significantly minimize your potential gain.
Is it Really About Principle?
"It's not the money; it is about principle". If that is the case, fine. Just remember it could be a very costly lesson in "principle". Can you stand on principle another way?
How Much Time Will it Take?
It may take a lot of your time. Factor this into the decision. If there are witnesses or businesses involved, it will take their time as well. This could strain relationships and have other personal costs.
Do a Cost/Benefit Analysis
Just to be sure it is worth it, do a cost benefit analysis to help you decide.