Depends on what is left. If it's something like 1/3 of the steak don't bother la. But generally these places have rather large portions so I already set aside a portion of it that I think I cannot finish (and I don't mess it up with sauce of whatever) and after the meal I ask to bungkus. I did that at TGIF the other day. Bungkus-ed one piece of chicken (it came in 2 pieces) and corn on the cob. They attended to my request as if it's the most normal thing to do.