Will G. S. wrote:
I plan on getting a new PSU fan and I'd like to avoid
doing any soldering, so I'm thinking about connecting
it to the motherboard, but I'm worried about leaving
the 2 pins on the PSU molex fan connector open and bare
because I fear that eventually dust would accumulate
and short the pins.

You shouldn't worry about shorts caused by dust. But it would be
uncouth to power the PSU fan from the motherboard. Soldering is not
very dangerous or expensive, so give it a try. Or just use butt
connectors, as others have recommended.