Ideally, you'll submit your manuscript in laser-printed form. If your dot-matrix printer has a pale ribbon and you can't replace it, make a darker photocopy of the original printout. Consider your choice of font. A boldface font is even worse. A serif font is more readable, so by all means choose one for the body of your manuscript text.
Point size is also important.
For the Mac, 12-point Times isn't bad, and it lets you put a lot of text on one page. But 14-point Times is more readable. I finally phoned Del Rey Books to see if they preferred a monospace font like Courier, or a more flexible font. If you use fanfold paper in a dot-matrix printer, make sure it's reasonably heavy. Give yourself a margin of at least an inch top and bottom, and an inch or an inch and a half on the sides.