I don't think a letter from a postdoc is bad, but a letter from a PI would be better if he or she knows you pretty well. In my lab, I've heard that sometimes a postdoc will write the letter and the PI will take it and look it over, make changes, etc. and will sign it. The higher up the chain of command, the better I would say, unless your PI wouldn't write you as good a recommendation or something like that.
