After many years of building planes (and a few other things) I developed a split outlook I will explain here.
I used to try to do everything perfectly. I'm pretty handy, and things generally came out pretty well, but .... not always as perfect as I envisioned.
There may have been a ding in the balsa under the covering, a blob of solder on the landing gear, an engine mount where a hole was stripped and one screw was larger than the others, or some trim finish that wasn't perfectly aligned.
Often when I finished, I would look at the plane (or whatever) and only see the problems, the spots where I might have done better.
Other people would look at it and see a straight, not-overly-heavy, nice-flying airplane, but all I would see was a mass of defects and wasn't happy.
Eventually, I saw the error of my ways. I found a quote recently from Dave Platt, the great Precision Scale R/C builder. He said that "Itís a mistake to take a scale model out to fly while you still like it." The scale models he's talking about are masterworks built for competition against other master builders, and it would be foolish to fly such a plane for pleasure.
For the rest of us who build planes in order to fly them, it would be foolish to spend the time and grief to try to make them look as nice as one of Dave's creations.
We build to fly, which means there's a very good chance it will crash at some point, and no matter how well-aligned your decals are, or how neat a soldering job you did on the landing gear, the plane will likely still crash, either because of a problem with "the nut behind the sticks" (by far the most common problem), or because of something completely unforseen, but never, ever, because of the minor flaws that had been spoiling my pleasure.
When building planes by gluing one piece of something to another piece of something else, nothing will ever be 100%: you could have sanded a little more (or less), you could have added a little more glue (or less), your paint could be a little bit better, and so on and so on. My advice is that unless you're building planes for competition, you should relax and enjoy yourself.
ON THE OTHER HAND, just because nothing will ever be really 100%, doesn't mean that you should aim for less. Good-looking planes fly better. Take your time and make sure your planes are straight, you'll have more fun with them. Parts that fit together well will require less glue and be a stronger assembly when put together, so take your time and sand them until they fit well.
If you aim for 100% and only make it to 98% things will still go well. On the other hand, if you aim for 90% and "chill" at 80% .... your planes will look more funky than they need to, you'll not get as much of the "warm, fuzzy feeling" that you should have, and that "inevitable" crash will be on you sooner rather than later.
Other articles and videos on this website show you ways to build well but also quickly, inexpensively and accurately, so click around, watch the videos and you'll probably find a few things you can use in your own builds.
And most of all, don't be sloppy, shoot for 100%, but, since we're here to have fun, don't stress about it if this one doesn't quite make it -- let your next plane be the perfect one (:-)