I'm 5'6 1/2" and 138 pounds this morning. I am wearing mostly 8s, a six and a really baggy pair of 9/10s in jeans right now. I'd like to think that at 135 I'll be comfy in my 8s and smoking in 6s. From there I'd like to see if I can get to 129 just to say I did and maintain around 130 but the 130s have been fighting me tooth and nail so we'll see. I would say I have a medium frame 34FF-29-37 with lots on top that won't go away. If I could have any weight I wanted it would be 125-130.
I haven't seen the 120s since the summer before high school when I grew 2" and C cups. If you are 5'6" the BMI chart lists 115 as the borderline for underweight and I would say that maintaining below 120 would be for someone who has a "small" frame. It is always on us to say "I look good, I feel good and the number is just a number." So I'm not going to say "don't do it that's too thin" because that is different for each of us, and you are the only one who can decide that. But I am going to say that I would have to nearly starve myself and work really hard to get under 120, I would have really pronounced hip bones and ribs (you can already see them a bit if I stretch my arms above my head

) and I probably still wouldn't get the tiny waist I want. So If you really are a medium frame then you might want to re-evaluate at 130.