In my driveway, I no longer use a spotter, done it too many times. With my 4 mirrors (2 stock plus towing mirrors), with the light incline I can see my tires clearly and now know exactly when to cut sharp to make the turn in the driveway so I can go park next to the garage. Kind of have to do a little S turn. I know before hand that the path is clear when I drive in front of the house before backing into the driveway.
When I back into a campsite, my wife will spot for me, but just to tell me when to stop. I pretty much have to rely on myself and the mirrors to line me up from side to side.
Now I backed up last week at our seasonal site, due to the layout, we have a 6 inch thick ramp on one side to make the TT level, and due to the proximity of the deck, I have to park on that ramp, and cannot move it (the ramp can be moved, but the trailer will not be where I want it to be). My friend was spotting me, he's very good, all I did was look at him through the mirror and did exactly as signaled. Still took 4 attempts because the ramp is the width of the tire, not more and also I wanted the trailer to be perfectly parallel to the deck, I'm anal that way.
In my opinion, a spotter is only needed if you're backing up in a blind spot or need extreme accuracy. It all depends on experience and comfort level. Professional drivers rarely use a spotter, but they have the experience.
When in doubt, use a spotter, if you don't have one, get out and look. I still get out and look to where I will back-up when it's in a new location.