Monospaced fonts are not monospaced

While this is not an answer, I did not want to add it as a comment. The issue is not specific to Linux, happens under Windows as well (Win7 in particular). Also, it is not specific to font: any monospaced font seems to produce overhangs to the default frame of $n$ characters. While rasterized characters have different image dimensions, as pointed out by halirutan, this is not direclty related to the problem, as most of the operator characters comply with the expected dimensions (though they will cause a lot of problems) while those characters that have larger-than-unit dimensions (like A, W, M) might fit in nicely without an overhang. So I think that the different image dimensions and the kerning problem of e.g. "_" are different issues.

1. Rasterized characters

For the image assembly, I would suggest using a uniform image dimension applied via ImageCrop:

(* get standard character dimensions - find a suitable char, like "I" *)
unitDim = ImageDimensions@
  Rasterize[Style["I", 30, Bold, FontFamily -> "Courier"], "Image"]

{18, 34}

"non cropped individual images"
ImageAssemble[
 Rasterize[Style[#, 30, Bold, FontFamily -> "Courier"], "Image"] & /@ 
  CharacterRange["A", "Z"]]

"cropped individual images"
ImageAssemble[
 ImageCrop[
    Rasterize[Style[#, 30, Bold, FontFamily -> "Courier"], "Image"], 
    unitDim] & /@ CharacterRange["A", "Z"]]

"rasterize as a whole"
Rasterize[
 Style[StringJoin[CharacterRange["A", "Z"]], 30, Bold, 
  FontFamily -> "Courier"], "Image"]

While cropping individual images is a possible workaround, sadly I have no idea why rasterized characters end up being larger than the monospaced frame. The following test indicates, that this effect depends on FontSize and Magnification but not on ImageResolution:

chars = CharacterRange["A", "Z"]; charPos = Thread[chars -> Range@Length@chars];
resRange = {10, 100, 1000}; sizeRange = {10, 15, 20, 30, 50, 100}; magRange = {.5, 1, 2, 10};

data = Table[
   style = {FontSize -> size, Magnification -> mag, FontWeight -> Bold, FontFamily -> "Courier"};
   img = Rasterize[Style[#, Sequence @@ style], "Image", ImageResolution -> res] & /@ chars;
   imgPos = Thread[img -> Range@Length@chars]; unitDim = ImageDimensions@img[["I" /. charPos]]; imgDim = ImageDimensions /@ img;
   Cases[img, x_?((d = ImageDimensions@#) =!= unitDim &) :> chars[[x /. imgPos]]],
   {res, resRange}, {size, sizeRange}, {mag, magRange}];

Column[MapThread[Labeled[TableForm[#1, TableHeadings -> {sizeRange, magRange}, TableDepth -> 2], "ImageResolution \[Rule] " <> ToString@#2, Top] &, {data, resRange}], Spacings -> 2]

Figure lists those rasterized characters that do not match the ImageDimension of the standard monospaced dimension (taken from "I"). Horizontal dimension: Magnification; Vertical dimension: FontSize. I would consider this a bug.

---

2. Monospaced characters are not monospaced

Further investigating the first issue of monospaced characters not being monospaced when printed:

n = 30;
font = "Courier"; (* alternatively: "Lucida Console" *)
text = StringJoin[
   RandomChoice[{"=", "*", ":"}, n], "\n",
   RandomChoice[{"M"}, n], "\n",
   RandomChoice[{"@"}, n], "\n",
   RandomChoice[{":"}, n], "\n",
   RandomChoice[{"M", "A"}, n], "\n",
   RandomChoice[{"_"}, n], "\n",
   RandomChoice[{"@", "M"}, n], "\n",
   RandomChoice[{":", "M"}, n], "\n",
   RandomChoice[{":", "A"}, n], "\n",
   RandomChoice[{":", "J"}, n]
   ];

Grid[{{
   Style[StringJoin["Normal, 100% magnification, font size 10\n", 
     text], FontFamily -> font, FontSize -> 10],
   Style[StringJoin["Bold, 100% magnification, font size 10\n", text],
     FontFamily -> font, FontSize -> 10, Bold]
  }, {
   Style[StringJoin["Normal, 120% magnification, font size 10\n", 
     text], FontFamily -> font, FontSize -> 10, Magnification -> 1.2],
   Style[StringJoin["Normal, 120% magnification, font size 12\n", 
     text], FontFamily -> font, FontSize -> 12, Magnification -> 1.2]
   }}, Alignment -> {Left, Top}, Frame -> All]

Note the followings:

• Non-bold monospaced font with 100% magnification and 10 points height produces nicely fit lines - any divergence causes overhangs.
• Operators with certain letters (while independently are ok) together form an overhang. This seems to be true for all the operators I've tested.
• "_" is a renegate character as it does not even fit by itself into the frame, not even when not Bold.
• Overhang depends on magnification, font family and font size.

Again, I have no idea what causes these problems, but is definitely operator-character-related. Here, the problem is that no setup that is found on one machine is guaranteed to work on another, as a lot of these parameters depend on the OS.