Should I be using an ice pack or heating pad 2 days post reduction?

There are complications associated with the use of both heat and cold during the early postoperative recovery period. These should be cleared by your surgeon before initiating either treatment. Your surgeon is your best resource as to the safety and timing when considering such intervention. 

I do not recommend heating pads or ice packs for swelling or pain alleviation following breast reduction.  Either can damage your skin as your sensitivity has changed, and you may not be able to differentiate hot from cold.Kenneth Hughes, MD, Board Certified Plastic SurgeonLos Angeles, CA

Generally ice is used in the early post operative setting to decrease swelling. Its important to be careful not to put ice directly onto the skin since you may not have enough sensation to tell you its been on too long and having negative effects. That being said you should check with your surgeon to see what they prefer for their post operative care. Good luck!