It depends, but I usually go for black text on a white background at least. It might be a leftover from my Macintosh days.
For older games, I tend to use monospaced fonts (e.g. simply running Frotz in a terminal window). The author may have expected it, and even in games that supported mixing proportional and monospaced text (e.g. the Infocom games), this was not always enforced as much as it perhaps should have been.
Some games clearly expect the interpreter to render emphasized text as italics (not underlined), so for those I seek out an interpreter that supports that. (Incidentally, Frotz recently started using italics by default when available.) More recent games probably expect that the interpreter will use a proportional font, so they are better at specifying when it shouldn’t.
But I usually don’t go so far as to specify exactly which font to use.
And of course, some games have no usable interpreters so I have to use emulation instead. Which limits my choices.