If you want to be "nicer" to the airline, sure, go ahead and cancel. United will then sell the same seat again - they'll like that. Should you care about making United Airlines Inc happy or not? Can't help you with that.
So your original cost of $300 is now gone. What can you recover? Can your flights be changed (not canceled)? If you try to reschedule, you (may?) get a credit for the original cost, minus some fee. How long do you have to utilize that credit (assuming you get some)?
If it's unlikely you can actually use it before it expires, could you make a change to just your return flight and turn this into a "mileage run"? Can you fly the original outbound leg, change the return flight to the same day you arrive, then just basically do a airport turnaround? That is, fly the outbound, get off the plane, stretch you legs, grab a bite to eat, hit the loo, then check in for the return flight and go straight home?
Sounds crazy but at least you would be credited with the miles for actually flying the round trip.
Granted, there are easier ways to generate a few thousand United miles, but if you've got nothing better to do that day...at least you might recover something of value (the miles).