Nope. Test your setup with everything sealed and closed, or you're out for a fatal water spill.
The 'small' hole is there in case you're filling the circuit from another point (in which case, you're *adding* water to the circuit thus the need to push air out at another point). Depending on how you plan to fill up (even through the res itself) it may help you prevent big 'gurgles' that usually happen - air escaping by the same route water is filling - so no water splash. But once it's filled, close it !
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