Steam can kill bed bugs at any phase if you use it right.  The quality of the steam cleaner is vital when using steam as a treatment.
Your surface temperature should reach 160-180 degrees F. Bed bugs can withstand temperatures below 160 degrees. When temperatures climb over 180 degrees, you could damage your sofa.
Hand-held steam cleaners have an infrared thermometer. It will help you measure surface temperatures. The rate at which you move the wand controls the temperature. When you move the wand slow, you'll get a low temperature. If it's too high, move the wand faster.