i used to use MS Paint, but now i use Adobe Photoshop and OCCASIONALLY MS Paint; my use of Paint these days is if i want to replace some colors that would take too long to do in Photoshop (mainly because i have all of the colors in one layer and borders in another). for example, if i wanted to replace Anglo-American Dark Red with English Pink, i'd probably just include a single pixel of it on one part of the map and continue working with the darker red for the rest of the map; once i'm done, i save the map as a PNG or bitmap and then use the eyedropper and eraser (yes, the eraser) to replace all of the original color with the new one
if you really look, you'll be able to see some of the maps i made solely in MS Paint back when i first joined AH.com. that was also before i transitioned to the uniform worlda maps and instead used a format i found on Wikipedia. most importantly, Photoshop makes it alot easier to work with adjusting borders and such because i can not only use layers but also move the pixels and separate them by layer so, if i fuck up, i can easily correct it (in contrast, on MS Paint you can only undo the last four things you did)