LÖVE

I went pretty much the opposite way. I used really low quality tiles, like 16x16, then upscaled them according to resolution. I choose from width / height the one that is lower for base value. For example lets say I am using 800x600 resolution, I would choose 600. Then I decide that I want to show at least 10 tiles for the shorter side of the screen, meaning in this case each tile would be 600 / 10 = 60 pixels in both width and height. Then I would show on the longer side 800 / 60 = 13 and something tiles. I am not sure if this is the better way of scaling or just making the tiles 13x10 then upscaling it to 800x600 better, but at least each tile is scaled the same way regardless of where in the screen the tile is. In other words I scale every tile instead of tlining first then scaling.
Also this means that depending on aspect ratio, the player might see more area on the longer side, and the square gives least amount of visibility.
On the otherhand the game that I am currently working on has no merits for having more visible area. And thus I even have zooming for the game. Even games like League of Legends at least used to give advantage for people using higher resolutions by showing more playing area on screen. (havent played the game for a year or two though)

edit: I first wrote wrong kind of method for what I am using, just some brain fart

micha wrote:I don't use pixel art. Originally I was aiming for a resolution of about 1280x800. If I determine the closest integer scaling factor, it will almost always be 1. And then I would have to accept that some people can see a larger part of the map than other, which I don't want.

you would probably have to use some sort of border then. instead of black borders, you could scale the HUD or whatever to fill the extra space.