I encountered low back strain due to bending down too low, last friday night. still a lot of pain at times. what is the general rule on ice vs. heat?

Varies somewhat: In general, if you have pulled a muscle and created a spasm; moist heat is particularly helpful to relieve; whereas ice tends to make people tighten up but can help to numb a superficial cause of pain. For some it is a little of trial and error.

Back Strain: Low back pain is a fact of life. Just about everybody will suffer from it sooner or later. One of the main causes of back pain, whether acute or chronic, is low back strain. http://www.webmd.com/back-pain/guide/low-back-strain