Water cooling issue

Hi guys ! I am a newbie at water cooling and I have a question after watching all the YouTube tutorials regarding the subject .

I want to fill the reservoir with the cooling liquid and I need to power my pc until a circuit is complete. All the tutorials show people inserting a wire in the 24 pin cable so you don't need to power the motherboard . Since my psu is guilt modular can I just leave the 24 pin cable unplugged from both its ends ??
 
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