i think placement matters a lot, i usually think for months about certain gaps and what will fill the spaces best, for a while i had two large tattoos on one leg and nothing on the other so i wasn't comfortable with it then i got one on the other to balance it out. but now i only have one knee done and i think knees like hands should be done as a pair. Since i was 16 id always wanted to have one arm black and grey and the other to be as colourful as possible and i finally finished my arms ten years later, i think it works really well as a contrast of colour and style.
im also saving my left leg as more of a collection of different artists but its been a coincidence so far and just so happens that thats where they've all been put.