Any leak can result in a loss of pressure in the cooling system and a loss of pressure or just insufficient pressure can have the hot coolant flash to steam.
UNLESS you use pure propylene glycol coolant with no water and no pressure in your cooling system, such as I do.
I used to run around without the caps even on the two reservoirs, but the blower reservoir hugs the hood, so it could slosh a bit on the padding. Now I just place them in there loosely to make sure not to pressure-cycle my system, which is a leading cause of failure in pressurized systems.
Aircraft bodies, for example, are given a certain number of pressure cycles as their lifespan.
If you want to convert painlessly, just take off both your caps and let the water boil off over time, replacing it with pure propylene glycol coolant when the levels drop, such as I did. The pink coolant IS propylene glycol. I didn't do it in hot weather, though, so chances of spot overheating were very minimal. The way I would suggest is short trips around town with no traffic. When you park it, do your shopping, and come back, look at levels in the reservoir. Let it drop quite low before putting in PG, but only add up to the usual fill line so the steam doesn't just push the PG out the overflow. The water will evaporate out over days for most of it, and weeks, for the few straggling molecules. My coolant levels have been stable for quite a few months, now.
No galvanic corrosion no matter how dissimilar and reactive the various engine parts are.
Now my car purrs like a kitten with no water slowly corroding the internals (PG is nonconductive) and no pressure cycling to deteriorate the hoses and resilient bits.
Also, no despised blanket boiling creating a layer of steam over the parts of the engine needing cooling the most, and "protecting" them from being cooled by the liquid coolant. Propylene glycol does not blanket boil, it nucleate boils, so it always has liquid hugging the cooled parts. Less cavitation at the water pump, too, due to its high, high boiling point (370 degrees F.)
Since it cools BETTER, the temp gauge may read hotter, but that is largely because it is just dragging more heat out of the hottest parts of the coolant jacket, such as the combustion chamber domes, that the water-based coolant used to "protect" from cooling by forming a protective blanket of insulating steam on top of them.
I'll never go back to water. Fifth vehicle with PG. One big-block, two small-blocks, an RX7, Neon, and this bambino.