It really depends on how far you're planning on running on the spare. I don't worry about a flat so much as the blow out! I tend to get out in BFE and a long ways from any tire places so I like to have all the tires the same. If I was just going a short distance I probably wouldn't care.
Build a man a fire and he's warm for the night. Light a man on fire and he's warm the rest of his life!