When I had to hang art in 10 hospitals in Pheonix last year, I used one from Home Desperate # $149.
It looks like a Lifeguards float, but that outside shell flowers open and becomes a tri-pod for the level. The downside was it only went up 4' so we stuck it in a shopping cart to get the 6' we needed.
Fennistrating all the tops to the same height is ok if you're doing same size pictures in a corperate setting ... but with random sizes in a gallery or home... I'd still go with the "general eye" level and two hooks.
Pros: They have almost all gotten very inexpensive ,
Cons: They are almost all made in China. (just MHO)