Not directly, but if it conflicts with the airline schedule, you will be in trouble. But in 121, you cannot work more then 100 hours a month, that includes instructing, even though it is not 121. you can fly all the hours you want as long as you're not being compensated for it. But if you instruct 15 hours a month and you reach 85 hours at your job and you can't fly anymore because of it, your airline may not be too happy with you. That is how I understand it from studying the FAR's.