Originally Posted by sfo1kmm
Is there any rule about using only one way (outbound) of a round trip ticket.
Just don't make a habit of it, and you're fine.
Sometimes you will be surprised how inexpensive a one-way can be nowadays, thanks to competition and "fare simplification". You might find one at or below a round-trip, as the airlines are willing to sell restricted one-ways now (not refundable and $100 change fees) for less then just offering a full-Y "extortion" fare. I am buying more one ways now when I know which leg I can confirm, and then buy the other half once my plans firm up.
Should you notify UA as a courtesy if you find that you will not be using the return portion?
No problem in doing so, but I would wait till the day before the flight departs.