Personally I just look out the drivers mirror when lining the tires up with the trailer, if that side is lined up, so is the other one. Make sure you are lined up before the rear axle goes under the trailer, so you don't have to turn once you start under. This is if you don't have fenders covering your tires, if you do you have to line the fenders up themselves, and it may take a few tries before you get used to exactly where to line them up.
Look in both side mirrors and center between the two. If one of the mirrors is pointing too low it can skew your perception.
It's just a matter of finding the right landmark on your tractor, as seen in your mirror, to line up with the trailer. I used the vertical line of the front drive tire on the driver-side and line it up with the vertical line of the trailer wall on the driver-side front corner. If the trailer was dropped properly as you back under the trailer your 5th wheel plate will raise the trailer and the landing gear feet won't be touching the ground. If the feet aren't touching the ground, there's no harm in being off-center, as long as the kingpin of the trailer is within the open wedge of the plate. There's is about a 99% chance the idiot driver who dropped the trailer you are picking up has dropped the trailer with landing gear too high. Not one person dropping trailers too high thinks they are dropping it too high. 99% of OTR drivers drop to high. Luckily, they seldom last a year before they are flipping burgers again.
Memorize the view of your driver-side outside front drive tire sidewall (imagine a yardstick vertical touching the sidewall) just as you slide away from trailer WHEN YOU ARE DROPPING THE TRAILER. Once you see the AFTER view, you'll know what the BEFORE should look like.
I wouldn't worry about it until the kingpin rips off the catwalk. Then you know you might be a little off center.
Once you think you are alinged, get out and crawl underneath. Make a line with the center hole of the frame and the center of the locking jaws. See if the kingpin is on that line. Odds are that your drives are centered, but the whole tractor is at a slight angle to the trailer. As you go backwards you ho from centered to off center. Angle to the trailer is the number one issue my guys have with getting coupled
Look only in your driver side mirror for backing. Also make sure your drive tires appear to be just outside the trailer as you back. Your drive tires may look to be offset, but in actuality, they will be lined up properly with the 5th wheel plate.
You can feel it hit the either side of the 5 wheel and slow your roll , 4 in or so over from the edge of the trailer drivers side , I dont like sliding the dollys sideways on a dropped nice new trailer loaded trailer it tweeks the landing gear , or it could get you hung up and stuck on ice and snow.