Having attend a few wakes (unfortunately), I have found them to be, as a rule, a little less formal. Oftentimes, you are only there for a few minutes, to pay your respects to the departed and his or her family. Depending on the size of the wake, there can be a considerable line, so everyone tries to keep it moving for the most part. Those closer to the family might stay for a longer period of time. Sometimes the priest will perform a small ceremony at the wake and, if you're there for the start of it, I feel it is rude to leave in the middle. All of the above being said, I always try to wear a jacket and tie (both in subdued colors), whether I am attending a wake or a funeral. I believe that this type of respect is warranted. You will see others who don't share my feelings.