I think as long as you don't send merit to your "Friend/Wife's account  " you are not doing anything wrong, however doing the right thing all the time is impossible,we all have our own method of choosing what posts to merit, but overall , the majority of those merit will go to whoever deserve them, you might get 5-10 merit for trolling or posting something funny, but all those merit combined don't really have that much of an impact on the accuracy of the merit system.

I also notice that as i "grow up" in the forum , I tend to change my preferences on what posts to merit, I recently started focusing on posts that open the door to discussion even if they don't have a direct answer to the problem/question in hand, because I personally believe that the forum in general has this problem of "good topics die way too fast", but things could change, next month I might think that the boards where I am most active lack new good topics so I will start focusing on new topics.