It depends on what other activities you plan on becomming involved with. Ex. If you are already on journal and moot court, being a TA is nice, but will likely take away from what little time you have. I was involved with all 3 and, while I certainly don't regret it, my firm didn't ask about the TA position at all during my interview - which leads me to believe it wasn't given much weight.
However, if you don't plan on being involved with anything else, being a TA certainly is worth the time. It will be a great way to open up an entire coversation with your interviewers when you are looking for a job.
ps. Interviewers could give a rats ass whether you had a personally convenient class schedule, so that should be the least of your concerns. Drop a class and become a TA.