The top flats of my barrels always look a little off, and how much and which direction depends on the angle I am viewing it from. Sometimes, it is my inletting, sometimes it is the barrel, usually, it is both. I have never checked the square on a barrel that was square all the way around from breech to muzzle. I buy good barrels and none of them are anywhere close to perfect as to the width or square of the flats. Frequently, the angle of the flats vary a little from breech to muzzle. The original barrels vary a lot more!
I inlet everything as square as possible, but nothing is ever perfect. As Eric as already indicated, nothing on a longrifle is perfectly square, perpendicular, parallel, or straight. The best you can do is keep looking at everything from different angles, using a raking light and do your best to end up with something that looks like a proper longrifle. That is why it is so important to look at and handle originals; because, in the end, the only criteria that matters is "does it look right?". Usually you have make stuff slightly off to make it look right.
I will tell you that I file the tang and breech with the stock, so , at least, the lock area should look right. The thing is, the trigger and trigger guard is almost never centered perfectly under the the lock panels due to cast-off (on). That means you have to fudge the whole area to make it kinda look right.
I go for an impression at arms length. When you start looking really close at any rifle, contemporary or antique (even the really good ones) you will find a lot of things wrong. The harder you look, the more you will find wrong. So, you are jut going for an impression, and the more guns you make, or the more time you spend on one, the better the final product is going to look from a distance.
The really fancy ones can hide a ton of small mistakes. One of the first things you learn about engraving is that you can disguise mistakes and imperfections by just adding more lines to confuse the eye. The more visual confusion, the more your brain will try to make things look the way it thinks they should look. In the end, you see what you think you should be seeing. Magicians would be out of business, otherwise.
So, bottom line; what we do is MAGIC!