You should definitely avoid them if you have clean Bitcoins. There are two reasons for that. First is they are very expensive. You will get much less out than you put in. Second is you might end up with Bitcoins someone pay to hire a murder with.
Should I really care? When you get paid with cash do you think about the things those bills were used to pay somewhere in the past? I sure don't and I believe you also don't do it.
Bitcoins are like cash. Some of the bills in your wallet probably came through the hands of drug dealers and it doesn't make them tainted. Why should mixed bitcoins be tainted?