Oven thermometers - accuracy

NOTE: Crossposted to r.f.cooking, r.f.baking, alt.bread.recipes; follow-ups set to r.f.equipment.
When I use my candy thermometer I calibrate it first ? I know that water boils at 212°F at my altitude. Last time I made candy (December) it was off by 7° and I adjusted accordingly; accuracy is important when making candy.
How do I know if my oven thermometer is accurate? How accurate does a baking temp have to be for baking bread?
My new thermometer (Taylor, non-gourmet-line product, bought at Target this morning for $6) says my oven is cool by 15 degrees. How do I know if that's accurate? I have no way to calibrate it that I know about. Is that (15 degrees off) going to make a difference in outcome if baking time isn't changed?
I'm trying to figure out if that's why my bread bottoms are a titch underdone. I like the color of the crust. Yeah, I'll bake a loaf with my oven set to 465° to see how that is but I'm curious right now.
Inquiring Mind Wants to Know.
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Melba's Jammin'

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