My system is as sealed as I can get it - no res (well - a T-fitting with a few inches of 3/8" ID Tygon for airtrap - VERY small res), lots of teflon-tape at the top of the air-trap where I screw a cap onto a barbed->threaded adapter in the top of the hose. Every other joint/barb/etc. is below fluid level and I don't have any leaks, so I assume they are sealed. Tygon is pretty stretchy stuff - the tubing can expand a bit if the pressure inside increases by a few drops (probably only 1 or 2 drops in my case as I have a very low volume of coolant).
Actually - to really test my system for leaks, I grabbed one section of tubing with both hands and squished it flat while the cap was on and the pump was off. This would have increased internal pressure by a LOT more than I will ever get from heat expansion, and I didn't even get a single drop of coolant leaking from any joints.
Even if the whole system was copper pipe and sealed, I think you'd need more than thermal expansion to break anything. You'd have a hard time getting so much air out of the system that there aren't a few bubbles somewhere that can't take a bit of compression.
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