Should you sump fuel tanks when the fuel system is below freezing?
OK, I sat on my hands for a while, so now I'll stick my head in the lion's mouth.

You should absolutely sump your tanks every time you go flying as part of a routine preflight inspection, whether you've taken on fuel or not, whether the aircraft has been hangared or out on the ramp, and even if it's below freezing.
Water isn't the only thing you're looking for (misfueling happens - also bits of fuel tank sealant, rubber from a rotting bladder, dirt that made it past the fueling truck's filters, etc. - you want to find these things!)

Ideally if it's below freezing you should also pre-heat your aircraft so the engine, instruments, and fuel are above freezing when you're preflighting (which is generally accomplished by sticking the plane in a heated hangar), but sometimes that's not an option and you make do.



Why should you sump the tanks if it's below freezing?

Well like I said above, water isn't the only thing we're looking for - and all those concerns still hold valid no matter what the temperature is. Water is the one that people seem to come up with as a reason for not sumping when it's below freezing though, so let's talk about water:  

There are five general ways water can manifest itself in your (Avgas) fuel system:


Liquid Water
If it's not been below freezing (or it's really sunny and your fuel tanks have acted like an oven) water may be liquid, waiting for you at the sump drain just like on a warm summer day. You'll drain it out and go on about your business.
Slush (ice crystals suspended in the fuel)
If it's below freezing you may have suspended ice crystals or slush in the fuel (particularly if you just took on fuel). This will look "cloudy" when you drain it from the sump and it's a good indication that today is not a good day to fly: you may have a LOT of water in the fuel, and you need to need to deal with it (warm the plane up and drain off the water, and figure out how so much of it got in there in the first place).
(There is a chance that slush could jam your fuel drain open, in which case your fuel tank will empty itself onto the ramp with great rapidity.)
Little bits of ice
Little drops of water can freeze into little bits of ice, which are often just too big to come out of the fuel sump drain, but just small enough to lodge in it and keep it from closing (which, again, will cause your tank to empty itself onto the ramp with great rapidity. This is also an indication that today is not a good day to fly).
BIG bits of ice over the sump drain
Liquid water will flow to the lowest point in the fuel system (the drains). If it's below freezing it will freeze there, potentially blocking the sump drain.
You'll notice this condition because the sump valve may feel stiff, your tank may not drain, or it will drain very slowly, possibly with water in the fuel.
This is a good indication that you shouldn't fly until you figure out what the cause of the problem is (it could be ice, it could be a defective valve, or it could be a bunch of other things, but until you know you want to stay on the ground).
Hidden Ice (big bits of ice away from the sump drain)
This is perhaps the most insidious: Ice that's not near the sump, and doesn't block or jam anything.  Hopefully you'll spot this sitting at the bottom of your tank when you look in to check the fuel level (you do visually check your fuel level, right?), but you probably won't notice anything out of the ordinary when you sump the tank.


(There are of course other ways water can hide out in the system besides the fuel tanks, but these are where you're most likely to find it.)

The most likely "bad thing" to happen to you in any of those scenarios is that your fuel sump will be jammed open by ice. This means you'll spill a tank's worth of avgas on the ground, and you'll have to call your airport's spill cleanup/emergency number and let them know.
That emergency is on the ground, and it may be a little embarrassing to have to explain to the spill cleanup folks, but it's better than the other possibilities.

The Bad Things that can happen to you in the air if you go flying with undetected ice/water in the fuel are far worse: The ice could melt and your engine could start trying to burn water, which doesn't ignite very well.

Check out Gene Benson's "Why Did They Do That? - Volume 2" presentation, starting at slide #7 for one example of how things can go catastrophically wrong if you neglect to sump your tanks (or ignore the warning signs when you do).